LeetCode Maximum Size Subarray Sum Equals k

原题链接在这里：https://leetcode.com/problems/maximum-size-subarray-sum-equals-k/

题目：

Given an array nums and a target value k, find the maximum length of a subarray that sums to k. If there isn't one, return 0 instead.

Example 1:

Given nums = [1, -1, 5, -2, 3], k = 3,
return 4. (because the subarray [1, -1, 5, -2] sums to 3 and is the longest)

Example 2:

Given nums = [-2, -1, 2, 1], k = 1,
return 2. (because the subarray [-1, 2] sums to 1 and is the longest)

Follow Up:
Can you do it in O(n) time?

题解：

用HashMap记录<sum, index> pair. 遇到sum-k在HashMap中时说明从hm.get(sum-k) 到 i 这一段的和是k.

Time Complexity: O(n). Space: O(n).

AC Java:

 1 public class Solution {
 2     public int maxSubArrayLen(int[] nums, int k) {
 3         if(nums == null || nums.length == 0){
 4             return 0;
 5         }
 6         HashMap<Integer,Integer> hm = new HashMap<Integer, Integer>();
 7         int sum = 0;
 8         int max = 0;
 9         hm.put(0,-1);
10         for(int i = 0; i<nums.length; i++){
11             sum += nums[i];
12             if(!hm.containsKey(sum)){
13                 hm.put(sum, i);
14             }
15             if(hm.containsKey(sum-k)){
16                 max = Math.max(max, i-hm.get(sum-k));
17             }
18         }
19         return max;
20     }
21 }