560. Subarray Sum Equals K

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

解題

func subarraySum(nums []int, k int) int {
res, sum := 0, 0
n := len(nums)
​
m := make(map[int]int) //紀錄前綴和出現次數
m[0] = 1
for i := 0; i < n; i++ {
sum += nums[i]
res += m[sum - k]
m[sum]++
}
​
return res
}