public int[] findDiagonalOrder(int[][] matrix) {
if (matrix == null || matrix.length == 0) {
int m = matrix.length, n = matrix[0].length;
int[] res = new int[m * n];
for (int i = 0; i < m * n; i++) {
res[i] = matrix[row][col];
if ((row + col) % 2 == 0) {