Coding interview questions
Common coding interview questions and the patterns behind them
Most coding interview problems are variations on a few patterns. Recognise the pattern and you are halfway there - then talk through the approach, the complexity and the edge cases.
1.Two Sum: find two numbers in an array that add up to a target.
- What they're checking
- Whether you go past the O(n²) brute force.
- How to answer
- Hash map: for each number, check whether target minus it has been seen. O(n) time, O(n) space.
2.Reverse a linked list.
- What they're checking
- Comfort with pointers.
- How to answer
- Walk the list with three pointers - previous, current, next - and flip each link. Mention the recursive version too.
3.Find the longest substring without repeating characters.
- What they're checking
- The sliding window pattern.
- How to answer
- Move a right pointer forward; when a character repeats, move the left pointer past its last position. Track the best length. O(n).
4.Check whether a string of brackets is balanced.
- What they're checking
- Using a stack.
- How to answer
- Push opening brackets; on a closing bracket, the top of the stack must match. The stack must be empty at the end.
5.Find the maximum depth of a binary tree.
- What they're checking
- Recursion on trees.
- How to answer
- Depth is 1 plus the larger depth of the two subtrees; an empty tree has depth 0. Mention the level-order (BFS) version.
6.Merge overlapping intervals.
- What they're checking
- Sorting as a first step.
- How to answer
- Sort by start; extend the last merged interval if the next one overlaps, otherwise start a new one. O(n log n).
7.Climbing stairs: how many ways to climb n steps, one or two at a time?
- What they're checking
- Spotting dynamic programming.
- How to answer
- Ways(n) = ways(n-1) + ways(n-2) - it's Fibonacci. Keep only the last two values for O(1) space.
8.Find the kth largest element in an array.
- What they're checking
- Heaps, and choosing between approaches.
- How to answer
- Keep a min-heap of size k: O(n log k). Mention quickselect for O(n) on average.
9.Count the islands in a grid of land and water.
- What they're checking
- Graph traversal on a grid.
- How to answer
- For each unvisited land cell, start a DFS or BFS that marks the whole island visited, and count the starts. O(rows × columns).
10.Design an LRU cache.
- What they're checking
- Combining data structures.
- How to answer
- A hash map for O(1) lookup plus a doubly linked list for recency order; move an item to the front on use and evict from the back.
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