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:

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

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My solution:
public class Solution {
public int[] findDiagonalOrder(int[][] matrix) {
if (matrix == null || matrix.length == 0) return new int[0];
int m = matrix.length;
int n = matrix[0].length;
int[] re = new int[m * n];
boolean flag = true;
int mnow = 0;
int nnow = 0;
for (int i = 0; i < m * n; ++i) {
re[i] = matrix[mnow][nnow];
if (flag) {
if (mnow == 0 || nnow == n - 1) {
if (nnow == n - 1)
mnow++;
else
nnow++;
flag = false;
} else {
mnow--;
nnow++;
}

} else {
if (mnow == m - 1 || nnow == 0) {
if (mnow == m - 1)
nnow++;
else
mnow++;
flag = true;
} else {
mnow++;
nnow--;
}
}
}
return re;
}
}

4000