| ✅ |
1 |
Two Sum |
Find two numbers in an array that sum to a target (use hashmap for O(1) lookups). |
| ✅ |
2 |
Reverse String |
Reverse a string in-place using two pointers (O(1) space). |
| ✅ |
3 |
Palindrome Number |
Check if a number reads the same backward (e.g., 121). Handle negatives. |
| ✅ |
4 |
Valid Parentheses |
Validate if brackets are correctly nested/closed (use a stack). |
| ✅ |
5 |
Merge Two Sorted Lists |
Combine two sorted linked lists into one (O(n) time, in-place). |
| ✅ |
6 |
Maximum Subarray |
Find the contiguous subarray with the largest sum (Kadane’s algorithm). |
| ✅ |
7 |
Climbing Stairs |
Count distinct ways to climb stairs (1 or 2 steps; Fibonacci pattern). |
| 🔲 |
8 |
Binary Search |
Search a target in a sorted array (O(log n) time). Handle edge cases. |
| 🔲 |
9 |
Linked List Cycle |
Detect if a linked list has a cycle (Floyd’s Tortoise and Hare algorithm). |
| 🔲 |
10 |
Valid Anagram |
Determine if two strings are anagrams (e.g., “listen” vs. “silent”). |
| 🔲 |
11 |
First Missing Positive |
Find the smallest missing positive integer in an array (O(n) time, O(1) space). |
| 🔲 |
12 |
Rotate Image |
Rotate an N×N matrix 90 degrees clockwise (in-place). |
| 🔲 |
13 |
Word Break |
Check if a string can be segmented into dictionary words (DP solution). |
| 🔲 |
14 |
Longest Increasing Subsequence |
Find the length of the longest subsequence (DP or patience sorting). |