560. Subarray Sum Equals K

标签（空格分隔）： leetcode array medium

---

题目

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 = 2Output: 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].

---

思路

这道题目我们一开始最简单的思路一定是平方算法，就是暴力计算下[i,j]之间的数值和。时间复杂度是O(N2),很明显这个会造成TLE。如果有人看过算法珠玑第八章的程序设计。一定会有这样的思路，就是计算出前i个元素的累加和。那么要判断[i,j]之间数值和就是计算cum[j]-cum[i-1]的数值是否为k。我们可以建立一个hashmap来存储cum，具体看代码。

代码

class Solution {public:    int subarraySum(vector<int>& nums, int k) {        unordered_map<int,int> presum;        int cum=0,count=0;        presum[0]++;        for(auto num:nums){            cum+=num;            count+=presum[cum-k];            presum[cum]++;        }        return count;    }};