LeetCode - Subarray Sum Equals K - 30Days Challenge

Given an array of integers and an integer k, you need to find the total number of continuous subarrays whose sum equals to k.

Example 1:

Input:nums = [1,1,1], k = 2 Output: 2

Note:

The length of the array is in range [1, 20,000].

The range of numbers in the array is [-1000, 1000] and the range of the integer k is [-1e7, 1e7].

Solution

Here we can take advantage of the Prefix Sum of the given array, Maintain prefix sum and check if prefixSum[i]-k is already visited before, If it is, that means we have sub-array whose sum is equal to k.

Code


import java.util.HashMap;
import java.util.Map;

public class SubArraySumK {

	public static void main(String[] args) {
		
		int [] a= {1,1,1};
		int k=2;
		
		System.out.println(new SubArraySumK().subarraySum(a, k));
	}

	public int subarraySum(int[] nums, int k) {
        int sum = 0, result = 0;
        Map<Integer, Integer> preSum = new HashMap<>();  // Maintains prefixSum and count of subArray ending with sum as k.
        preSum.put(0, 1);
        
        for (int i = 0; i < nums.length; i++) {
            sum += nums[i];
            if (preSum.containsKey(sum - k)) {
                result += preSum.get(sum - k);
            }
            preSum.put(sum, preSum.getOrDefault(sum, 0) + 1);
        }
        
        return result;
    }
}