325. Maximum Size Subarray Sum Equals k

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.

Note:
The sum of the entire nums array is guaranteed to fit within the 32-bit signed integer range.

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?

本题和Minimum Size Subarray Sum有点类似之处，那道题的nlongn的做法里面，使用了二分查找，这里面的target和那道题的target比较类似，但是并不是用二分查找做出来的，而是存储一个hashmap来寻找，代码如下：

public class Solution {
    public int maxSubArrayLen(int[] nums, int k) {
        Map<Integer,Integer> map = new HashMap<Integer,Integer>();
        int sum = 0;
        int len = 0;
        for(int i=0;i<nums.length;i++){
            sum+=nums[i];
            if(sum==k) len = i+1;
            else if(map.containsKey(sum-k)) len=Math.max(len,i-map.get(sum-k));
            if(!map.containsKey(sum)) map.put(sum,i);
        }
        return len;
    }
}