560. 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].

int subarraySum(vector<int>& nums, int k) { // time: O(n); space: O(n)
    unordered_map<int, int> preSum;
    ++preSum[0];
    int sum = 0, res = 0;
    for (int num : nums) {
        sum += num;
        if (preSum.count(sum - k)) res += preSum[sum - k];
        ++preSum[sum];
    }
    return res;
}