【LeetCode 560】 Subarray Sum Equals K

題目描述

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

哈希表記錄前綴和出現的次數。

代碼

class Solution {
public:
    int subarraySum(vector<int>& nums, int k) {
        int n = nums.size();
        int cur = 0, res = 0;
        unordered_map<int, int> counts{{0, 1}};
        for (int num : nums) {
            cur += num;
            res += counts[cur-k];
            counts[cur]++;
        }
        return res;
    }
};