Max Sum Without Adjacent Elements

Given a 2 x N grid of integer, A, choose numbers such that the sum of the numbers
is maximum and no two chosen numbers are adjacent horizontally, vertically or diagonally, and return it.

Note: You can choose more than 2 numbers.

The first and the only argument of input contains a 2d matrix, A.

Return an integer, representing the maximum possible sum.

1 <= N <= 20000
1 <= A[i] <= 2000

Input 1:
    A = [   [1]
            [2]    ]

Output 1:
    2

Explanation 1:
    We will choose 2.

Input 2:
    A = [   [1, 2, 3, 4]
            [2, 3, 4, 5]    ]
    
Output 2:
    We will choose 3 and 5.

Method:

For each column, we choose the biggest number, and then add it with the previous result from Max(col - 1, col - 2)

Solution:

Time: O(n)
Space: O(1)

public class Solution {
    public int adjacent(int[][] A) {
        if (A == null || A.length == 0) return 0;
        int prevColOne = 0;
        int prevColTwo = 0;
        int colOne = 0;
        int colTwo = 0;
        for (int j = 0; j < A[0].length; j ++) {
            if (j % 2 == 0) {
                prevColOne = colOne;
                colOne = Math.max(prevColTwo, colOne) + Math.max(A[0][j], A[1][j]);
            } else {
                prevColTwo = colTwo;
                colTwo = Math.max(prevColOne, colTwo) + Math.max(A[0][j], A[1][j]);
            }
        }
        return Math.max(colOne, colTwo);
    }
}