Interview Q&A / Data Structures & Algorithms

1. How do you count subarrays whose sum equals k using a prefix sum and hash map?

   Keep a running prefix sum and a map of how many times each prefix sum has occurred. At each step, the number of subarrays ending here with sum k equals the count of earlier prefix sums equal to current minus k.

   IntermediateQuick answerCoding1 min read
2. How do you find the length of the longest consecutive sequence in an unsorted array in O(n)?

   Put all numbers in a set. Only start counting from a number whose predecessor is not in the set, then walk upward while the next number exists. Each number is visited at most twice.

   IntermediateQuick answerCoding1 min read
3. How do you group anagrams together from a list of words?

   Anagrams share the same sorted letters, so use the sorted word as a hash map key and collect each word under its key. The map values are the groups.

   IntermediateQuick answerCoding1 min read