[leetcode]Map-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].

 

class Solution {
public:
    int subarraySum(vector<int>& nums, int k) {
        int n=nums.size();
        if (n==0)
            return 0;
       
        vector<int> sum(n+1,0);
        for (int i=1; i<=n; i++)
            sum[i]=nums[i-1]+sum[i-1];
        
        unordered_map<int,int> hash;
        hash[0]=1;
        int res=0;
        for (int i=1; i<=n; i++)
        {
            if (hash[sum[i]-k])
                res+=hash[sum[i]-k];
            hash[sum[i]]++;
        }
        return res;
    }
};