Small example
Input
nums = [1,1,1], k = 2
Output
2
Subarrays [1,1] at indices (0,1) and (1,2).
Full lesson preview
Count Subarrays with Sum k Using Prefix Hash Map
Count the number of continuous subarrays whose sum equals k using prefix sums and a hash map.
Python practice16 minHashing & SetsIntermediateLast updated March 26, 2026
Problem statement
Given an integer array nums and an integer k, return the number of continuous subarrays whose sum equals to k. Use a hashmap to store prefix sum frequencies: for current prefix sum 'presum', the number of valid subarrays ending at current index equals the frequency of (presum - k).
Task
Apply prefix-sum and hashing to count subarrays summing to k in O(n) time and O(n) space.
Examples
Input format
An integer list nums and an integer k.
Output format
An integer count of continuous subarrays whose sum equals k.
Constraints
-10^5 <= nums[i] <= 10^5, 0 <= len(nums) <= 10^5. Use O(n) time and O(n) extra space.
Samples
Sample 1
Input
nums = [1,2,3], k = 3
Output
2
Subarrays [1,2] and [3].