LeetCode - 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

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].

解法一：利用sum之间的差暴力搜索

class Solution {
    public int subarraySum(int[] nums, int k) {
        if (nums == null || nums.length == 0)
            return 0;
        int len = nums.length;
        int[] sum = new int[len+1];
        for (int i=0; i<len; i++)
            sum[i+1] = sum[i] + nums[i];
        int cnt = 0;
        for (int i=0; i<len; i++) {
            for (int j=i+1; j<=len; j++) {
                if (sum[j] - sum[i] == k)
                    cnt ++;
            }
        }
        return cnt;
    }
}

解法二：利用map建立sum和出现次数cnt的映射关系

class Solution {
    public int subarraySum(int[] nums, int k) {
        if (nums == null || nums.length == 0)
            return 0;
        Map<Integer, Integer> map = new HashMap<>();
        map.put(0, 1);
        int sum = 0, cnt = 0;
        for (int num : nums) {
            sum += num;
            cnt += map.getOrDefault(sum-k, 0);
            map.put(sum, map.getOrDefault(sum, 0)+1);
        }
        return cnt;
    }
}

第二种解法耗时更少