DSA/Subarray_Sum_Equals_K.java at main · adi-gp/DSA · GitHub

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/*

Given an array of integers nums and an integer k, return the total number of subarrays whose sum equals to k.

A subarray is a contiguous non-empty sequence of elements within an array.


Example 1:

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

Input: nums = [1,2,3], k = 3
Output: 2


Constraints:

1 <= nums.length <= 2 * 104
-1000 <= nums[i] <= 1000
-107 <= k <= 107

*/
import java.util.*;
class Subarray_Sum_Equals_K {
    public int subarraySum(int[] nums, int k) {
        int prefix = 0;
        int count = 0;
        HashMap<Integer, Integer> map = new HashMap<>();
        map.put(0,1);
        for(int i =0 ; i<nums.length ; i++){
            prefix = prefix + nums[i];
            int need = prefix - k ;
            if(map.containsKey(need)){
                count = count + map.get(need);
            }
            map.put(prefix , map.getOrDefault(prefix , 0)+1);
        }
        return count ;
    }
}