[Array] 560Subarray sum Equal 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

Solution

方法1：暴力解法

for (int i=0;i<size;++i){
    sum = 0;
    for(int j=i;j<size;++j){
        sum+=nums[j];
        if (sum==k) count ++;
    }
}

方法2：考虑从第0位到第k位的总和为sum[k], 如果有sum[i]-sum[j] == k，那么从i+1到j位即为符合要求到子序列。

class Solution {
public:
    int subarraySum(vector<int>& nums, int k) {
        int size = nums.size();
        if (size==0) return 0;
        int sum=0, count=0;
        map<int, int> map1;
        map1.insert(pair<int, int>(0, 1));

        for(int i=0;i<size;i++){
            sum += nums[i];
            auto iter = map1.find(sum-k);
            if (iter != map1.end()){
                count += map1[sum-k];
            }

            auto iter1 = map1.find(sum);
            if (iter1 != map1.end()){
                map1[sum] ++ ;
            }
            else{
                map1[sum] = 1;
            }
        }

        return count;
    }
};