325 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)

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

Solutions:

public class Solution {
    public int maxSubArrayLen(int[] nums, int k) {
        int max = 0;
        for (int i = 0; i < nums.length; i ++) {
            int sum = 0;
            for (int j = i; j < nums.length; j ++) {
                sum += nums[j];
                if (sum == k) {
                    max = Math.max(max, j - i + 1);
                }
            }
        }
        return max;
    }
}

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