【leetcode】【medium】【每日一題】560. Subarray Sum Equals K

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

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].

題目鏈接：https://leetcode-cn.com/problems/subarray-sum-equals-k/

 

思路

常用套路：前綴和，計算下標0～i的元素之和，但記錄時是由和 查找前綴個數。

對於 當前數值nums[i] 和 當前和sum，如果存在 sum-前綴和j=k，則說明j～i段爲所求連續子串。

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