leetcode 560. Subarray Sum Equals K

题目

https://leetcode.com/problems/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

Constraints:

The length of the array is in range [1, 20,000].
The range of numbers in the array is [-1000, 1000] and the range of the integer k is [-1e7, 1e7].

思路

记录向前累计和,而后向前查找距离目标k的差值出现多少次,区间问题尽量转换为向前累计和的操作,而不是区间和的问题

源码

class Solution {
public:
    unordered_map<int,int> m;
    int sum[20005];
    int res = 0;
    int subarraySum(vector<int>& nums, int k) {
        sum[0] = 0;
        m[0] = 1;
        for(int i = 1;i <= nums.size();++i){
            int s = sum[i-1]+nums[i-1];
            sum[i] = s;
            int re = s-k;
            if(m.find(re)!=m.end()){
                res+=m[re];
            }
            if(m.find(s)==m.end()){
                m[s]=1;
            }else{
                m[s]++;
            }
        }
        return res;
    }
};

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