---
description: Prefix Sum
---

# LeetCode 560. Subarray Sum Equals K

Given an array of integers `nums` and an integer `k`, return _the total number of continuous subarrays whose sum equals to `k`_.

**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`

## Solution

[English Version in Youtube](https://youtu.be/l0dLq-qI588)

[中文版解答Youtube Link](https://youtu.be/cBGP5zAzom0)

[中文版解答Bilibili Link](https://www.bilibili.com/video/BV1aK4y1U7jb/)

```
class Solution {
public:
    int subarraySum(vector<int>& nums, int k) {
        unordered_map<int, int> prefix_sum;
        prefix_sum[0] = 1;
        
        int sum = 0;
        int ans = 0;
        for (int num : nums) {
            sum += num;
            int target = sum - k;
            ans += prefix_sum[target];
            prefix_sum[sum]++;
        }
        
        return ans;
    }
};
```
