# 560. Subarray Sum Equals K (Medium)

https://leetcode.com/problems/subarray-sum-equals-k/

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:

1. The length of the array is in range [1, 20,000].
2. The range of numbers in the array is [-1000, 1000] and the range of the integer k is [-1e7, 1e7].

## Solutions

``````class Solution {

// The clever solution si to work the accumulated sums from preceding elements.
// Then keep count of the occurrences for a particular sum. To make it more specifically,
// nums[1,3,4,5,-5], countMap take turns add/update (0,1),(1,1),(4,1),(8,1),(13,1),(8,2)

public int subarraySum(int[] nums, int k) {
int ans = 0;
if (nums == null || nums.length == 0) {
return ans;
}

Map<Integer, Integer> countMap = new HashMap<>();

// At the very beginning, no element summed and the initial sum should be 0,
// the occurrence is 1.
countMap.put(0, 1);

int sum = 0;
for (int i = 0; i < nums.length; i++) {
sum += nums[i];

// if the value (sum-k) used to be the sum of prior subarray. For instance,
// assume current index is y and sum-k=3, sum of subarray [0:x] is 3, then we
// could take subarray [x+1:y] as one of the answers.

if (countMap.containsKey(sum - k)) {
ans += countMap.get(sum - k);
}

if (!countMap.containsKey(sum)) {
countMap.put(sum, 0);
}

countMap.put(sum, countMap.get(sum) + 1);
}

return ans;
}
}
``````