Google coding & dsa interview questions

Sample questions with short answers

Tap a question to reveal the short answer and key points.

1. 1Given a stream of integers, design a structure that returns the median at any time.MediumGoogle-specificShow short answer

   Short answer

   Keep two heaps: a max-heap for the lower half and a min-heap for the upper half, rebalanced so sizes differ by at most one. Insert is O(log n); the median is the top of the larger heap or the average of both tops. Google interviewers expect you to state the invariant clearly and handle the even/odd size cases.

   Interviewers listen for

   • two heaps for lower and upper halves
   • rebalance so sizes differ by at most one
   • O(log n) insert and O(1) median
2. 2Find the longest substring with at most K distinct characters.HardGoogle-specificShow short answer

   Short answer

   Use a sliding window with a map of character counts. Expand the right edge; when the map has more than K keys, shrink from the left until it has K again, updating the best length on every step. O(n) time, O(K) space. Google rounds reward talking through why the window never needs to move backwards.

   Interviewers listen for

   • sliding window with character counts
   • shrink from the left when the map has more than K keys
   • O(n) time
3. 3Given an array of integers and a target, return the indices of the two numbers that add up to the target. How would you approach it and what is the complexity?EasyAsked everywhereShow short answer

   Short answer

   Walk the array once keeping a hash map of value → index. For each element check whether target − value is already in the map; if so return both indices. This is O(n) time and O(n) space, versus the O(n²) brute-force double loop. Clarify whether exactly one solution exists and whether the same element can be reused.

   Interviewers listen for

   • hash map of value to index
   • walk the array once, O(n) time
   • clarify duplicates and reuse of an element
4. 4How do you detect a cycle in a linked list without using extra memory?MediumAsked everywhereShow short answer

   Short answer

   Use Floyd's tortoise-and-hare: advance a slow pointer by one node and a fast pointer by two. If the fast pointer reaches null there is no cycle; if the pointers ever meet there is one. It runs in O(n) time with O(1) extra space. To find the cycle's start, reset one pointer to the head and move both one step at a time until they meet again.

   Interviewers listen for

   • slow and fast pointers
   • O(1) extra space
   • pointers meeting proves a cycle
5. 5Explain the difference between BFS and DFS and when you would choose each.MediumAsked everywhereShow short answer

   Short answer

   BFS explores level by level using a queue, so it finds the shortest path in an unweighted graph and suits 'fewest steps' problems. DFS goes deep first using a stack or recursion, uses less memory on wide graphs, and suits connectivity, cycle detection, topological sort and backtracking. Both are O(V + E); pick by what the problem asks for and the graph's shape.

   Interviewers listen for

   • BFS uses a queue and finds shortest paths
   • DFS uses a stack or recursion
   • both O(V + E)