Leetcode: 子数组总和等于K(Subarray Sum Equals K)（C++）

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].

给定一个整数数组和一个整数k，您需要找到总和等于k的连续子数组的总数。

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凡是有求和的题目的化简似乎都需要弄个和的数组

这边我们使用哈希map来存储之前的和的数量，这样我们就能快速地找到sum(0,i).sum(i,j)=sum(0,j)-sum(0,i)，从而找到相应的数值。

class Solution {
public:
    int subarraySum(vector<int>& nums, int k) {
        unordered_map<int, int> sum_table;
        int sum = 0, buffer = 0, result = 0;
        sum_table.insert(make_pair(0, 1));
        for(int i = 0; i < nums.size(); i++){
            sum += nums[i];
            buffer = sum - k;
            auto it = sum_table.find(buffer);
            if(it != sum_table.end()){
                result += it->second;
            }
            it = sum_table.find(sum);
            if(it != sum_table.end()){
                it->second++;
            }
            else{
                sum_table.insert(make_pair(sum, 1));
            }
        }
        return result;
    }
};