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?

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