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:

Input: nums = [1, -1, 5, -2, 3], k = 3
Output: 4
Explanation: The subarray [1, -1, 5, -2] sums to 3 and is the longest.
Example 2:

Input: nums = [-2, -1, 2, 1], k = 1
Output: 2
Explanation: The subarray [-1, 2] sums to 1 and is the longest.
Follow Up:
Can you do it in O(n) time?

Solution

Prefix Sum + HashMap

sumi - k = sumj

Complexity

O(N) O(N)

Code

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