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# 560. Subarray Sum Equals K

## LeetCode [560. Subarray Sum Equals K](https://leetcode-cn.com/problems/subarray-sum-equals-k/)

### Description

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 * 10^4`
* `-1000 <= nums[i] <= 1000`
* `-10^7 <= k <= 10^7`

### Tags

Array, Hash Table

### Solution

![](/files/fwzT0z55Q5tAm2naPEoD)

This is a classic prefix-sum problem. To obtain the left and right bound of a subarray whose sum is `k`, we can find 2 prefix-sum who meet the condition $$pre\[i]−pre\[j−1]==k$$.

In terms of implementation, we build a map `m` whose key is sum of first `n` numbers and value is the occurrences of this sum, initialized with `{0: 1}`. Then we iterate over `nums`, and:

1. calculate prefix-sum `preSum`;
2. update the counter of *subarrays whose sum equals to`k`*, by adding `m[preSum-k]` onto it;
3. add `m[preSum]` by 1.

### Complexity

* Time complexity: $$O(n)$$
* Space complexity: $$O(n)$$

### Code

```go
func subarraySum(nums []int, k int) int {
	ans, preSum, m := 0, 0, map[int]int{0: 1}
	for _, num := range nums {
		preSum += num
		ans += m[preSum-k]
		m[preSum]++
	}
	return ans
}
```

## Reference

1. [和为K的子数组](https://leetcode-cn.com/problems/subarray-sum-equals-k/solution/he-wei-kde-zi-shu-zu-by-leetcode-solution/)


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