**Leetcode 560. Subarray Sum Equals K

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

总觉得结合数据结构降低时间复杂度的题有点没意思。但是既然是比较长问的题还是准备下。。。。

参考了 https://leetcode.com/problems/subarray-sum-equals-k/discuss/102121/C++-prefix-sum-+-map

先ans += 后Rec[cur]++ 因为k = 0的情况

class Solution {
public:
    int subarraySum(vector<int>& nums, int k) {
        unordered_map<int, int> rec;
        int ans = 0, cur = 0;
        rec[0]++;
        for (int i = 0; i < nums.size(); i++) {
            cur += nums[i];
            ans += rec[cur-k];
            rec[cur] ++;
            
        }
        return ans;
    }
};