560. Subarray Sum Equals K

转载于 2018-04-24 08:26:10 发布 · 159 阅读

本文介绍了一种高效算法，用于找出整数数组中所有连续子数组中和等于特定整数K的数量。该算法利用累积和的概念及哈希表记录前缀和出现次数，从而在O(n)的时间复杂度内解决问题。

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:

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].

class Solution {
public:
    int subarraySum(vector<int>& nums, int k) {
        int cum=0; // cumulated sum
        map<int,int> rec; // prefix sum recorder
        int cnt = 0; // number of found subarray
        rec[0]++; // to take into account those subarrays that begin with index 0
        for(int i=0;i<nums.size();i++){
            cum += nums[i];
            cnt += rec[cum-k];
            rec[cum]++;
        }
        return cnt;
    }
};