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- 01-matrix
- 1-bit-and-2-bit-characters
- 132-pattern
- 2-keys-keyboard
- 24-game
- 3sum-closest
- 3sum-smaller
- 3sum-with-multiplicity
- 3sum
- 4-keys-keyboard
- 4sum-ii
- 4sum
- a-number-after-a-double-reversal
- abbreviating-the-product-of-a-range
- accepted-candidates-from-the-interviews
- account-balance
- accounts-merge
- active-businesses
- active-users
- activity-participants
- actors-and-directors-who-cooperated-at-least-three-times
- ad-free-sessions
- add-and-search-word-data-structure-design
- add-binary
- add-bold-tag-in-string
- add-digits
- add-minimum-number-of-rungs
- add-one-row-to-tree
- add-strings
- add-to-array-form-of-integer
- add-two-numbers-ii
- add-two-numbers
- add-two-polynomials-represented-as-linked-lists
- adding-spaces-to-a-string
- adding-two-negabinary-numbers
- additive-number
- ads-performance
- advantage-shuffle
- airplane-seat-assignment-probability
- alert-using-same-key-card-three-or-more-times-in-a-one-hour-period
- alien-dictionary
- all-divisions-with-the-highest-score-of-a-binary-array
- all-elements-in-two-binary-search-trees
- all-nodes-distance-k-in-binary-tree
- all-oone-data-structure
- all-paths-from-source-lead-to-destination
- all-paths-from-source-to-target
- all-people-report-to-the-given-manager
- all-possible-full-binary-trees
- all-the-pairs-with-the-maximum-number-of-common-followers
- all-valid-triplets-that-can-represent-a-country
- allocate-mailboxes
- alphabet-board-path
- ambiguous-coordinates
- amount-of-new-area-painted-each-day
- analyze-user-website-visit-pattern
- android-unlock-patterns
- angle-between-hands-of-a-clock
- apples-oranges
- apply-discount-every-n-orders
- arithmetic-slices-ii-subsequence
- arithmetic-slices
- arithmetic-subarrays
- armstrong-number
- arranging-coins
- array-nesting
- array-of-doubled-pairs
- array-partition-i
- array-transformation
- array-with-elements-not-equal-to-average-of-neighbors
- article-views-i
- article-views-ii
- as-far-from-land-as-possible
- assign-cookies
- asteroid-collision
- available-captures-for-rook
- average-height-of-buildings-in-each-segment
- average-of-levels-in-binary-tree
- average-salary-departments-vs-company
- average-salary-excluding-the-minimum-and-maximum-salary
- average-selling-price
- average-time-of-process-per-machine
- average-waiting-time
- avoid-flood-in-the-city
- backspace-string-compare
- bag-of-tokens
- balance-a-binary-search-tree
- balanced-binary-tree
- bank-account-summary-ii
- bank-account-summary
- base-7
- baseball-game
- basic-calculator-ii
- basic-calculator-iii
- basic-calculator-iv
- basic-calculator
- battleships-in-a-board
- beautiful-arrangement-ii
- beautiful-arrangement
- beautiful-array
- before-and-after-puzzle
- best-meeting-point
- best-position-for-a-service-centre
- best-sightseeing-pair
- best-team-with-no-conflicts
- best-time-to-buy-and-sell-stock-ii
- best-time-to-buy-and-sell-stock-iii
- best-time-to-buy-and-sell-stock-iv
- best-time-to-buy-and-sell-stock-with-cooldown
- best-time-to-buy-and-sell-stock-with-transaction-fee
- best-time-to-buy-and-sell-stock
- big-countries
- biggest-single-number
- biggest-window-between-visits
- binary-gap
- binary-number-with-alternating-bits
- binary-prefix-divisible-by-5
- binary-search-tree-iterator-ii
- binary-search-tree-iterator
- binary-search-tree-to-greater-sum-tree
- binary-search
- binary-searchable-numbers-in-an-unsorted-array
- binary-string-with-substrings-representing-1-to-n
- binary-subarrays-with-sum
- binary-tree-cameras
- binary-tree-coloring-game
- binary-tree-inorder-traversal
- binary-tree-level-order-traversal-ii
- binary-tree-level-order-traversal
- binary-tree-longest-consecutive-sequence-ii
- binary-tree-longest-consecutive-sequence
- binary-tree-maximum-path-sum
- binary-tree-paths
- binary-tree-postorder-traversal
- binary-tree-preorder-traversal
- binary-tree-pruning
- binary-tree-right-side-view
- binary-tree-tilt
- binary-tree-upside-down
- binary-tree-vertical-order-traversal
- binary-tree-zigzag-level-order-traversal
- binary-trees-with-factors
- binary-watch
- bitwise-and-of-numbers-range
- bitwise-ors-of-subarrays
- boats-to-save-people
- bold-words-in-string
- bomb-enemy
- boundary-of-binary-tree
- brace-expansion-ii
- brace-expansion
- break-a-palindrome
- brick-wall
- bricks-falling-when-hit
- brightest-position-on-street
- broken-calculator
- buddy-strings
- build-an-array-with-stack-operations
- build-array-from-permutation
- build-array-where-you-can-find-the-maximum-exactly-k-comparisons
- build-binary-expression-tree-from-infix-expression
- build-the-equation
- building-boxes
- building-h2o
- buildings-with-an-ocean-view
- bulb-switcher-ii
- bulb-switcher-iii
- bulb-switcher-iv
- bulb-switcher
- bulls-and-cows
- burst-balloons
- bus-routes
- calculate-money-in-leetcode-bank
- calculate-salaries
- calculate-special-bonus
- camelcase-matching
- campus-bikes-ii
- campus-bikes
- can-convert-string-in-k-moves
- can-i-win
- can-make-arithmetic-progression-from-sequence
- can-make-palindrome-from-substring
- can-place-flowers
- can-you-eat-your-favorite-candy-on-your-favorite-day
- candy-crush
- candy
- capacity-to-ship-packages-within-d-days
- capital-gainloss
- capitalize-the-title
- car-fleet-ii
- car-fleet
- car-pooling
- card-flipping-game
- cat-and-mouse-ii
- cat-and-mouse
- cells-with-odd-values-in-a-matrix
- chalkboard-xor-game
- champagne-tower
- change-minimum-characters-to-satisfy-one-of-three-conditions
- change-the-root-of-a-binary-tree
- cheapest-flights-within-k-stops
- check-array-formation-through-concatenation
- check-completeness-of-a-binary-tree
- check-if-a-number-is-majority-element-in-a-sorted-array
- check-if-a-parentheses-string-can-be-valid
- check-if-a-string-can-break-another-string
- check-if-a-string-contains-all-binary-codes-of-size-k
- check-if-a-string-is-a-valid-sequence-from-root-to-leaves-path-in-a-binary-tree
- check-if-a-word-occurs-as-a-prefix-of-any-word-in-a-sentence
- check-if-all-1s-are-at-least-length-k-places-away
- check-if-all-as-appears-before-all-bs
- check-if-all-characters-have-equal-number-of-occurrences
- check-if-all-the-integers-in-a-range-are-covered
- check-if-an-original-string-exists-given-two-encoded-strings
- check-if-array-is-sorted-and-rotated
- check-if-array-pairs-are-divisible-by-k
- check-if-binary-string-has-at-most-one-segment-of-ones
- check-if-every-row-and-column-contains-all-numbers
- check-if-it-is-a-good-array
- check-if-it-is-a-straight-line
- check-if-move-is-legal
- check-if-n-and-its-double-exist
- check-if-number-is-a-sum-of-powers-of-three
- check-if-numbers-are-ascending-in-a-sentence
- check-if-one-string-swap-can-make-strings-equal
- check-if-string-is-a-prefix-of-array
- check-if-string-is-decomposable-into-value-equal-substrings
- check-if-string-is-transformable-with-substring-sort-operations
- check-if-the-sentence-is-pangram
- check-if-there-is-a-valid-path-in-a-grid
- check-if-two-expression-trees-are-equivalent
- check-if-two-string-arrays-are-equivalent
- check-if-word-can-be-placed-in-crossword
- check-if-word-equals-summation-of-two-words
- check-if-word-is-valid-after-substitutions
- check-whether-two-strings-are-almost-equivalent
- checking-existence-of-edge-length-limited-paths-ii
- checking-existence-of-edge-length-limited-paths
- cherry-pickup-ii
- cherry-pickup
- choose-numbers-from-two-arrays-in-range
- cinema-seat-allocation
- circle-and-rectangle-overlapping
- circular-array-loop
- circular-permutation-in-binary-representation
- classes-more-than-5-students
- climbing-stairs
- clone-binary-tree-with-random-pointer
- clone-graph
- clone-n-ary-tree
- closest-binary-search-tree-value-ii
- closest-binary-search-tree-value
- closest-dessert-cost
- closest-divisors
- closest-leaf-in-a-binary-tree
- closest-room
- closest-subsequence-sum
- clumsy-factorial
- coin-change-2
- coin-change
- coin-path
- coloring-a-border
- combination-sum-ii
- combination-sum-iii
- combination-sum-iv
- combination-sum
- combinations
- combine-two-tables
- compare-strings-by-frequency-of-the-smallest-character
- compare-version-numbers
- complement-of-base-10-integer
- complete-binary-tree-inserter
- complex-number-multiplication
- concatenated-words
- concatenation-of-array
- concatenation-of-consecutive-binary-numbers
- confirmation-rate
- confusing-number-ii
- confusing-number
- connecting-cities-with-minimum-cost
- consecutive-available-seats
- consecutive-characters
- consecutive-numbers-sum
- consecutive-numbers
- constrained-subsequence-sum
- construct-binary-search-tree-from-preorder-traversal
- construct-binary-tree-from-inorder-and-postorder-traversal
- construct-binary-tree-from-preorder-and-inorder-traversal
- construct-binary-tree-from-preorder-and-postorder-traversal
- construct-binary-tree-from-string
- construct-k-palindrome-strings
- construct-quad-tree
- construct-string-from-binary-tree
- construct-target-array-with-multiple-sums
- construct-the-lexicographically-largest-valid-sequence
- construct-the-rectangle
- contain-virus
- container-with-most-water
- contains-duplicate-ii
- contains-duplicate-iii
- contains-duplicate
- contiguous-array
- continuous-subarray-sum
- convert-1d-array-into-2d-array
- convert-a-number-to-hexadecimal
- convert-binary-number-in-a-linked-list-to-integer
- convert-binary-search-tree-to-sorted-doubly-linked-list
- convert-bst-to-greater-tree
- convert-date-format
- convert-integer-to-the-sum-of-two-no-zero-integers
- convert-sorted-array-to-binary-search-tree
- convert-sorted-list-to-binary-search-tree
- convert-to-base-2
- convex-polygon
- coordinate-with-maximum-network-quality
- copy-list-with-random-pointer
- corporate-flight-bookings
- correct-a-binary-tree
- count-all-possible-routes
- count-all-valid-pickup-and-delivery-options
- count-and-say
- count-apples-and-oranges
- count-binary-substrings
- count-common-words-with-one-occurrence
- count-complete-tree-nodes
- count-different-palindromic-subsequences
- count-elements-with-strictly-smaller-and-greater-elements
- count-fertile-pyramids-in-a-land
- count-good-meals
- count-good-nodes-in-binary-tree
- count-good-numbers
- count-good-triplets
- count-items-matching-a-rule
- count-largest-group
- count-negative-numbers-in-a-sorted-matrix
- count-nice-pairs-in-an-array
- count-nodes-equal-to-sum-of-descendants
- count-nodes-with-the-highest-score
- count-number-of-homogenous-substrings
- count-number-of-maximum-bitwise-or-subsets
- count-number-of-nice-subarrays
- count-number-of-pairs-with-absolute-difference-k
- count-number-of-special-subsequences
- count-number-of-teams
- count-numbers-with-unique-digits
- count-odd-numbers-in-an-interval-range
- count-of-matches-in-tournament
- count-of-range-sum
- count-of-smaller-numbers-after-self
- count-operations-to-obtain-zero
- count-pairs-in-two-arrays
- count-pairs-of-equal-substrings-with-minimum-difference
- count-pairs-of-nodes
- count-pairs-with-xor-in-a-range
- count-primes
- count-salary-categories
- count-servers-that-communicate
- count-sorted-vowel-strings
- count-special-quadruplets
- count-square-submatrices-with-all-ones
- count-square-sum-triples
- count-student-number-in-departments
- count-sub-islands
- count-subarrays-with-more-ones-than-zeros
- count-submatrices-with-all-ones
- count-substrings-that-differ-by-one-character
- count-substrings-with-only-one-distinct-letter
- count-subtrees-with-max-distance-between-cities
- count-the-hidden-sequences
- count-the-number-of-consistent-strings
- count-the-number-of-experiments
- count-the-repetitions
- count-triplets-that-can-form-two-arrays-of-equal-xor
- count-unhappy-friends
- count-unique-characters-of-all-substrings-of-a-given-string
- count-univalue-subtrees
- count-vowel-substrings-of-a-string
- count-vowels-permutation
- count-ways-to-build-rooms-in-an-ant-colony
- count-ways-to-distribute-candies
- count-ways-to-make-array-with-product
- count-words-obtained-after-adding-a-letter
- counting-bits
- counting-elements
- countries-you-can-safely-invest-in
- couples-holding-hands
- course-schedule-ii
- course-schedule-iii
- course-schedule-iv
- course-schedule
- cousins-in-binary-tree
- cracking-the-safe
- crawler-log-folder
- create-a-session-bar-chart
- create-maximum-number
- create-sorted-array-through-instructions
- create-target-array-in-the-given-order
- critical-connections-in-a-network
- custom-sort-string
- customer-order-frequency
- customer-placing-the-largest-number-of-orders
- customer-who-visited-but-did-not-make-any-transactions
- customers-who-bought-all-products
- customers-who-bought-products-a-and-b-but-not-c
- customers-who-never-order
- cut-off-trees-for-golf-event
- cutting-ribbons
- cyclically-rotating-a-grid
- daily-leads-and-partners
- daily-temperatures
- data-stream-as-disjoint-intervals
- day-of-the-week
- day-of-the-year
- decode-string
- decode-the-slanted-ciphertext
- decode-ways-ii
- decode-ways
- decode-xored-array
- decode-xored-permutation
- decoded-string-at-index
- decompress-run-length-encoded-list
- decrease-elements-to-make-array-zigzag
- decrypt-string-from-alphabet-to-integer-mapping
- deepest-leaves-sum
- defanging-an-ip-address
- defuse-the-bomb
- degree-of-an-array
- delete-and-earn
- delete-characters-to-make-fancy-string
- delete-columns-to-make-sorted-ii
- delete-columns-to-make-sorted-iii
- delete-columns-to-make-sorted
- delete-duplicate-emails
- delete-duplicate-folders-in-system
- delete-leaves-with-a-given-value
- delete-n-nodes-after-m-nodes-of-a-linked-list
- delete-node-in-a-bst
- delete-node-in-a-linked-list
- delete-nodes-and-return-forest
- delete-operation-for-two-strings
- delete-the-middle-node-of-a-linked-list
- delete-tree-nodes
- delivering-boxes-from-storage-to-ports
- department-highest-salary
- department-top-three-salaries
- depth-of-bst-given-insertion-order
- describe-the-painting
- design-a-file-sharing-system
- design-a-leaderboard
- design-a-stack-with-increment-operation
- design-add-and-search-words-data-structure
- design-an-expression-tree-with-evaluate-function
- design-an-ordered-stream
- design-authentication-manager
- design-bitset
- design-bounded-blocking-queue
- design-browser-history
- design-circular-deque
- design-circular-queue
- design-compressed-string-iterator
- design-excel-sum-formula
- design-file-system
- design-front-middle-back-queue
- design-hashmap
- design-hashset
- design-hit-counter
- design-in-memory-file-system
- design-linked-list
- design-log-storage-system
- design-most-recently-used-queue
- design-movie-rental-system
- design-parking-system
- design-phone-directory
- design-search-autocomplete-system
- design-skiplist
- design-snake-game
- design-tic-tac-toe
- design-twitter
- design-underground-system
- destination-city
- destroying-asteroids
- detect-capital
- detect-cycles-in-2d-grid
- detect-pattern-of-length-m-repeated-k-or-more-times
- detect-squares
- determine-color-of-a-chessboard-square
- determine-if-string-halves-are-alike
- determine-if-two-strings-are-close
- determine-whether-matrix-can-be-obtained-by-rotation
- detonate-the-maximum-bombs
- di-string-match
- diagonal-traverse-ii
- diagonal-traverse
- diameter-of-binary-tree
- diameter-of-n-ary-tree
- dice-roll-simulation
- diet-plan-performance
- different-ways-to-add-parentheses
- digit-count-in-range
- dinner-plate-stacks
- display-table-of-food-orders-in-a-restaurant
- distance-between-bus-stops
- distant-barcodes
- distinct-echo-substrings
- distinct-numbers-in-each-subarray
- distinct-subsequences-ii
- distinct-subsequences
- distribute-candies-to-people
- distribute-candies
- distribute-coins-in-binary-tree
- distribute-repeating-integers
- divide-a-string-into-groups-of-size-k
- divide-array-in-sets-of-k-consecutive-numbers
- divide-array-into-increasing-sequences
- divide-chocolate
- divide-two-integers
- divisor-game
- domino-and-tromino-tiling
- dot-product-of-two-sparse-vectors
- dota2-senate
- drop-type-1-orders-for-customers-with-type-0-orders
- dungeon-game
- duplicate-emails
- duplicate-zeros
- earliest-possible-day-of-full-bloom
- edit-distance
- egg-drop-with-2-eggs-and-n-floors
- element-appearing-more-than-25-in-sorted-array
- elements-in-array-after-removing-and-replacing-elements
- eliminate-maximum-number-of-monsters
- elimination-game
- employee-bonus
- employee-free-time
- employee-importance
- employees-earning-more-than-their-managers
- employees-whose-manager-left-the-company
- employees-with-missing-information
- encode-and-decode-strings
- encode-and-decode-tinyurl
- encode-n-ary-tree-to-binary-tree
- encode-number
- encode-string-with-shortest-length
- equal-rational-numbers
- equal-sum-arrays-with-minimum-number-of-operations
- equal-tree-partition
- erect-the-fence-ii
- erect-the-fence
- escape-a-large-maze
- escape-the-ghosts
- evaluate-boolean-expression
- evaluate-division
- evaluate-reverse-polish-notation
- evaluate-the-bracket-pairs-of-a-string
- even-odd-tree
- exam-room
- excel-sheet-column-number
- excel-sheet-column-title
- exchange-seats
- exclusive-time-of-functions
- execution-of-all-suffix-instructions-staying-in-a-grid
- expression-add-operators
- expressive-words
- factor-combinations
- factorial-trailing-zeroes
- fair-candy-swap
- falling-squares
- fancy-sequence
- faulty-sensor
- fibonacci-number
- filling-bookcase-shelves
- filter-restaurants-by-vegan-friendly-price-and-distance
- final-prices-with-a-special-discount-in-a-shop
- final-value-of-variable-after-performing-operations
- find-a-corresponding-node-of-a-binary-tree-in-a-clone-of-that-tree
- find-a-peak-element-ii
- find-a-value-of-a-mysterious-function-closest-to-target
- find-all-anagrams-in-a-string
- find-all-duplicates-in-an-array
- find-all-good-strings
- find-all-groups-of-farmland
- find-all-lonely-numbers-in-the-array
- find-all-numbers-disappeared-in-an-array
- find-all-people-with-secret
- find-all-possible-recipes-from-given-supplies
- find-all-the-lonely-nodes
- find-anagram-mappings
- find-and-replace-in-string
- find-and-replace-pattern
- find-array-given-subset-sums
- find-bottom-left-tree-value
- find-center-of-star-graph
- find-common-characters
- find-critical-and-pseudo-critical-edges-in-minimum-spanning-tree
- find-cumulative-salary-of-an-employee
- find-customer-referee
- find-customers-with-positive-revenue-this-year
- find-cutoff-score-for-each-school
- find-distance-in-a-binary-tree
- find-duplicate-file-in-system
- find-duplicate-subtrees
- find-elements-in-a-contaminated-binary-tree
- find-eventual-safe-states
- find-first-and-last-position-of-element-in-sorted-array
- find-first-palindromic-string-in-the-array
- find-followers-count
- find-good-days-to-rob-the-bank
- find-greatest-common-divisor-of-array
- find-if-path-exists-in-graph
- find-in-mountain-array
- find-interview-candidates
- find-k-closest-elements
- find-k-length-substrings-with-no-repeated-characters
- find-k-pairs-with-smallest-sums
- find-k-th-smallest-pair-distance
- find-kth-bit-in-nth-binary-string
- find-kth-largest-xor-coordinate-value
- find-largest-value-in-each-tree-row
- find-latest-group-of-size-m
- find-leaves-of-binary-tree
- find-longest-awesome-substring
- find-lucky-integer-in-an-array
- find-median-from-data-stream
- find-median-given-frequency-of-numbers
- find-minimum-in-rotated-sorted-array-ii
- find-minimum-in-rotated-sorted-array
- find-minimum-time-to-finish-all-jobs
- find-missing-observations
- find-mode-in-binary-search-tree
- find-n-unique-integers-sum-up-to-zero
- find-nearest-point-that-has-the-same-x-or-y-coordinate
- find-nearest-right-node-in-binary-tree
- find-numbers-with-even-number-of-digits
- find-original-array-from-doubled-array
- find-peak-element
- find-permutation
- find-pivot-index
- find-positive-integer-solution-for-a-given-equation
- find-right-interval
- find-root-of-n-ary-tree
- find-servers-that-handled-most-number-of-requests
- find-smallest-common-element-in-all-rows
- find-smallest-letter-greater-than-target
- find-subsequence-of-length-k-with-the-largest-sum
- find-substring-with-given-hash-value
- find-target-indices-after-sorting-array
- find-the-celebrity
- find-the-city-with-the-smallest-number-of-neighbors-at-a-threshold-distance
- find-the-closest-palindrome
- find-the-derangement-of-an-array
- find-the-difference
- find-the-distance-value-between-two-arrays
- find-the-duplicate-number
- find-the-highest-altitude
- find-the-index-of-the-large-integer
- find-the-kth-largest-integer-in-the-array
- find-the-kth-smallest-sum-of-a-matrix-with-sorted-rows
- find-the-longest-substring-containing-vowels-in-even-counts
- find-the-longest-valid-obstacle-course-at-each-position
- find-the-middle-index-in-array
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- find-the-student-that-will-replace-the-chalk
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- finding-mk-average
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- integer-break
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- investments-in-2016
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- keyboard-row
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- largest-1-bordered-square
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- list-the-products-ordered-in-a-period
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- long-pressed-name
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- magic-squares-in-grid
- magical-string
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- 1180 entries not shown
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2083. Substrings That Begin and End With the Same Letter (Medium)

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Hint 1

In the string "abacad", the letter "a" appears 3 times. How many substrings begin with the first "a" and end with any "a"?

Hint 2

There are 3 substrings ("a", "aba", and "abaca"). How many substrings begin with the second "a" and end with any "a"? How about the third?

Hint 3

2 substrings begin with the second "a" ("a", and "aca") and 1 substring begins with the third "a" ("a").

Hint 4

There is a total of 3 + 2 + 1 = 6 substrings that begin and end with "a".

Hint 5

If a character appears i times in the string, there are i * (i + 1) / 2 substrings that begin and end with that character.

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