LeetCode 560 Subarray Sum Equals K (hash)

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].

题目链接：https://leetcode.com/problems/subarray-sum-equals-k/

题目分析：前缀和，hashmap，没了

12ms，时间击败99.86%

class Solution {
    
    public int subarraySum(int[] nums, int k) {
        HashMap<Integer, Integer> mp = new HashMap<>();
        int[] sum = new int[nums.length + 1];
        for (int i = 1; i <= nums.length; i++) {
            sum[i] = sum[i - 1] + nums[i - 1];
        }
        int ans = 0;
        for (int i = 0; i < sum.length; i++) {
            if (mp.containsKey(sum[i] - k)) {
                ans += mp.get(sum[i] - k);
            }
            mp.put(sum[i], mp.getOrDefault(sum[i], 0) + 1);
        }
        return ans;
    }
}