This repository contains all the leetcode must solve problems.
Before jumping into coding:
- Read the problem twice.
- Identify input and output formats.
- Clarify constraints (e.g., input size, time limits).
- Restate the problem in your own words.
- Think of examples manually.
Key Questions to Ask:
- What exactly do I need to find/return?
- Are the inputs sorted, unique, or repetitive?
- Do I care about time/memory efficiency?
| Problem Type | Hints for Data Structures/Algorithms |
|---|---|
| Searching | Binary Search, Hash Maps, Trees |
| Sorting | Merge Sort, Quick Sort |
| String Manipulation | Two Pointers, Stack, Sliding Window |
| Dynamic Programming | Arrays, Recursion + Memoization |
| Graph Problems | BFS, DFS, Dijkstra, Union-Find |
| Range Queries | Segment Trees, Fenwick Trees |
| Combinatorics | Recursion, Backtracking |
| Data Structure | Use When You Need To... |
|---|---|
| Array/List | Store a sequence; access by index |
| HashMap | Quick lookup by key (O(1) avg. time) |
| Set | Store unique items quickly |
| Stack | LIFO behavior; backtracking, parsing |
| Queue | FIFO behavior; BFS, task scheduling |
| Heap | Get min/max quickly; priority queues |
| Graph | Represent connections/networks |
| Tree | Hierarchical data, faster search (BST, Trie) |
| Pattern | Description | Common Problems |
|---|---|---|
| Two Pointers | Iterate from both ends | Palindromes, Sorted Arrays |
| Sliding Window | Subarrays with conditions | Max sum subarray |
| Binary Search | Divide and conquer search | Search in rotated array |
| DFS/BFS | Graph/tree traversal | Path finding |
| Backtracking | Try all options and undo | Sudoku, N-Queens |
| Greedy | Choose best option locally | Activity selection |
| Dynamic Prog. | Break into subproblems | Fibonacci, Knapsack |
- Input:
"leetcode" - Output:
"l"
- Understand: Return the first character that appears only once.
- Example:
"loveleetcode"→"v" - Constraints: Only lowercase letters.
- Approach: Use a HashMap to count frequencies.
- Traverse string, count frequency.
- Traverse again, return first character with frequency 1.
#include <iostream>
#include <unordered_map>
#include <string>
char firstNonRepeatingChar(const std::string& s) {
std::unordered_map<char, int> freq;
// Count frequencies
for (char ch : s) {
freq[ch]++;
}
// Find first non-repeating
for (char ch : s) {
if (freq[ch] == 1) {
return ch;
}
}
return '\0'; // or a special char like '_' if no such character
}
int main() {
std::string input = "loveleetcode";
char result = firstNonRepeatingChar(input);
if (result != '\0')
std::cout << "First non-repeating character: " << result << std::endl;
else
std::cout << "No non-repeating character found." << std::endl;
return 0;
}This README serves as a cheat-sheet and includes:
- Problem understanding checklist ✅
- Classification of problem types 🔍
- Key data structures and patterns 🧰
- Real example solved step-by-step with C++ 💻
| 0020-valid-parentheses |
| 1078-remove-outermost-parentheses |
| 1737-maximum-nesting-depth-of-the-parentheses |
| 0013-roman-to-integer |
| 0048-rotate-image |
| 0189-rotate-array |
| 0268-missing-number |
| 2032-largest-odd-number-in-string |
| 0011-container-with-most-water |
| 2032-largest-odd-number-in-string |
| 0014-longest-common-prefix |
| 0812-rotate-string |
| 0015-3sum |
| 0049-group-anagrams |
| 0075-sort-colors |
| 0088-merge-sorted-array |
| 0169-majority-element |
| 0217-contains-duplicate |
| 0242-valid-anagram |
| 0268-missing-number |
| 0451-sort-characters-by-frequency |
| 0451-sort-characters-by-frequency |
| 0451-sort-characters-by-frequency |
| 0169-majority-element |
| 0451-sort-characters-by-frequency |
| 1890-sum-of-beauty-of-all-substrings |
| 0005-longest-palindromic-substring |
| 0053-maximum-subarray |
| 0118-pascals-triangle |
| 0121-best-time-to-buy-and-sell-stock |
| 0152-maximum-product-subarray |
| 0136-single-number |
| 0268-missing-number |
| 0053-maximum-subarray |
| 0169-majority-element |
| 0048-rotate-image |
| 0054-spiral-matrix |
| 0073-set-matrix-zeroes |
| 0054-spiral-matrix |
| 0238-product-of-array-except-self |
| 0560-subarray-sum-equals-k |
| 0021-merge-two-sorted-lists |
| 0021-merge-two-sorted-lists |
| 0003-longest-substring-without-repeating-characters |
| 0076-minimum-window-substring |
| 0424-longest-repeating-character-replacement |