Maximum Size Subarray Sum Equals k

Problem:

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)

Analysis:

Typical presum and hashmap problem. Note that if we found current sum in hashmap don't update the index, since the existing one is further. 

Solution:

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