LeetCode - 498. Diagonal Traverse

Given a matrix of M x N elements (M rows, N columns), return all elements of the matrix in diagonal order as shown in the below image.

Example:

Input:
[
 [ 1, 2, 3 ],
 [ 4, 5, 6 ],
 [ 7, 8, 9 ]
]
Output:  [1,2,4,7,5,3,6,8,9]
Explanation:

Note:

1. The total number of elements of the given matrix will not exceed 10,000.

class Solution {
    public int[] findDiagonalOrder(int[][] matrix) {
        boolean up = true;
        if (matrix.length == 0) return new int[0];
        int[] res = new int[matrix.length * matrix[0].length];
        int i = 0;
        int j = 0;
        for (int k = 0; k < matrix.length * matrix[0].length; k++) {
            res[k] = matrix[i][j];
            if (up) {
                if ((i-1) >= 0 && (j+1) < matrix[0].length) {
                    i--;
                    j++;
                } else if ((j+1) < matrix[0].length) {
                    j++;
                    up = false;
                } else if ((i+1) < matrix.length) {
                    i++;
                    up = false;
                } else break;
            } else {
                if ((i+1) < matrix.length && (j-1) >= 0) {
                    i++;
                    j--;
                } else if ((i+1) < matrix.length) {
                    i++;
                    up = true;
                } else if ((j+1) < matrix[0].length) {
                    j++;
                    up = true;
                } else break;
            }
        }
        return res;

    }
}