LeetCode

LeetCode Problems & Solutions.

#	Title	Solution	Topic	Basic Idea
1	Two Sum	Python	Array, Hash Table	HashTable: While scanning through the array, wait for the target - x using hash map
15	3Sum	Python	Two Pointers, Sorting	4 Cases exist: [0,0,0], [0,p,-p], [p1,p2,-(p1+p2)], [n1, n2, -(n1+n2)]
22	Generate Parentheses	Python	DP, Backtracking, Stack	Stack: stack=[("(", l, r)]
45	Jump Game II	Python	DP, Greedy	DP: O(n^2), Greedy: (TBD)
46	Permutations	Python	Backtracking	Backtrack: Add nums[i] -> Go next -> Pop nums[i] with visited flag
55	Jump Game	Python	DP, Greedy	Greedy: (Forward) if cur position > max position then False (Backward) if last position can reach the first index then True
62	Unique Paths	Python	Combinatorics, DP	Combinatorics: (m+n-2)C(m-1), DP: dp[i][j]=dp[i-1][j]+dp[i][j-1]
70	Climbing Stairs	Python	DP, Math	DP: dp[i]=dp[i-1]+dp[i-2] where dp[0]=1
71	Simplify Path	Python	String, Stack	Stack: .. (pop), . or empty (ignore), else (push)
77	Combinations	Python	Backtracking	Backtrack: Add i -> Go next -> Backtrack (=pop i)
78	Subsets	Python	Backtracking, Bit Manipulation	Cascading: Append nums[i] (doubling the size of previous one) Backtrack: Add nums[i] -> Go next -> Backtrack (=pop nums[i])
90	Subsets II	Python	Backtracking, Bit Manipulation	Backtrack: Same idea as #78 + set.add(tuple(sorted(list))) to remove duplicates
101	Symmetric Tree	Python	DFS, BFS, Stack	DFS(Recursive): dfs(l.l, r.r) and dfs(l.r, r.l) Stack(Iterative): Basically, mirror idea is SAME & return False when one is None or vals are different
112	Path Sum	Python	BinarySearch, DFS, BFS	BFS: (root, acc_sum) DFS: (targetSum-root.val) for left and right, recursively
113	Path Sum II	Python	DFS, BFS, Backtracking	BFS: (root, [root.val])
172	Factorial Trailing Zeroes	Python	Math	O(N): dp[i]=i//5 + dp[i//5] if i%5==0 O(logN): Add # of 5s and then # of 25s ... until it reaches N
200	Number of Islands	Python	DFS, BFS, UnionFind	DFS: dfs(x+dx, y+dy) BFS: queue=[(x,y)]
221	Maximal Square	Python	DP	DP: DP[i][j]=min(DP[i][j-1], DP[i-1][j], DP[i-1][j-1])+1
231	Power of Two	Python	Bit Manipulation	BitManipulation: (2^n) and (2^n)-1 are always complementary
287	Find the Duplicate Number	Python	Two Pointers, Binary Search, Bit Manipulation	TwoPointers:
322	Coin Change	Python	DP, BFS	DP: dp[i]=min(dp[i-coins[0]], dp[i-coins[1]], dp[i-coins[2]], ... )+1
326	Power of Three	Python	Math, NumberTheory	Math: math.log(n, 3), NumberTheory: Check max pow(3)%n
342	Power of Four	Python	Bit Manipulation, Math	Math: math.log(n, 4)
437	Path Sum III	Python	DFS, BFS	(TBD)
518	Coin Change 2	Python	DP	DP: dp[i]+=dp[i-coin] (Key idea: For-loop-coin-first-then-amount)
1051	Height Checker	Python	Sort, CountingSort	Sort: 1-Liner using zip & sort
1200	Minimum Absolute Difference	Python	Sorting	Sort and find the min with zip (arr, arr[1:])
1268	Search Suggestions System	Python	Trie, BinarySearch	BinarySearch: sort and bisect_left Trie: (TBD)
1306	Jump Game III	Python	DFS, BFS	BFS: q=[index] and A[index]=-1 DFS: return A[index]==0 or f(index+A[index]) or f(index-A[index])
1512	Number of Good Pairs	Python	HashTable, Math, Counting	Math: set and count (but it requires modification of the given array), HashTable: Frequency Table is all you need
2000	Reverse Prefix of Word	Python	TwoPointers, String	String: index and [::-1]

Categories: Dynamic Programming (DP), BFS, DFS, Math, Stack, Queue, Hash Table, Back Tracking, Graph

Key Point

1. Backtrack: Find all the possible cases for the given situation
2. DP:
3. BFS:
4. DFS:

Tips

• index vs find
• set vs list

Reference

• LeetHub: Linking GitHub and LeetCode