5. Arrays
Traversal, in-place updates, rotation and the top array interview questions.
Your progress0 of 14
7easy
6medium
1hard
Understand the topic
An array is a row of numbered lockers: you can open any locker instantly by its number (O(1)), but inserting into the middle means shifting everyone after it (O(n)).
Most array problems are won by one of these moves: a single pass with a running value (max, min, count), using the values themselves as indexes (marking or "cyclic sort"), reversing parts of the array, or writing from the back so you don't overwrite data you still need.
| # | Problem | Pattern | Level | ||
|---|---|---|---|---|---|
| 1 | Build Array from Permutation #1920 | Index mapping | Easy | LeetCode ↗ | |
| 2 | Max Consecutive Ones #485 | Running counter | Easy | LeetCode ↗ | |
| 3 | Check if Array Is Sorted and Rotated #1752 | Count drops | Easy | LeetCode ↗ | |
| 4 | Best Time to Buy and Sell Stock #121 | Running minimum | Easy | LeetCode ↗ | |
| 5 | Find All Numbers Disappeared in an Array #448 | Index marking | Easy | LeetCode ↗ | |
| 6 | Plus One #66 | Carry handling | Easy | LeetCode ↗ | |
| 7 | Merge Sorted Array #88 | Merge from the back | Easy | LeetCode ↗ | |
| 8 | Rotate Array #189 | Reversal trick | Medium | LeetCode ↗ | |
| 9 | Next Permutation #31 | Pivot + reverse | Medium | LeetCode ↗ | |
| 10 | Rearrange Array Elements by Sign #2149 | Two write indexes | Medium | LeetCode ↗ | |
| 11 | Find All Duplicates in an Array #442 | Index marking | Medium | LeetCode ↗ | |
| 12 | Find the Duplicate Number #287 | Floyd's cycle on indexes | Medium | LeetCode ↗ | |
| 13 | H-Index #274 | Counting sort | Medium | LeetCode ↗ | |
| 14 | First Missing Positive #41 | Cyclic sort | Hard | LeetCode ↗ |
Nothing matches that filter.
Work top to bottom — each list goes from easy to hard. Try a problem for at least 15 minutes before opening a hint. Ticks are saved in your browser.