Spiral Matrix

Given a matrix of m x n elements (m rows, n columns), return all elements of the matrix in spiral order.

For example,
Given the following matrix:

[
 [ 1, 2, 3 ],
 [ 4, 5, 6 ],
 [ 7, 8, 9 ]
]

 1 public class Solution {
 2     public List<Integer> spiralOrder(int[][] matrix) {
 3         List<Integer> result = new ArrayList<>();
 4         if (matrix == null || matrix.length == 0) {
 5             return result;
 6         }
 7         int m = matrix.length;
 8         int n = matrix[0].length;
 9         if (m == 1) {
10             for (int i = 0; i < n; i++) {
11                 result.add(matrix[0][i]);
12             }
13             return result;
14         }
15         if (n == 1) {
16             for (int i = 0; i < m; i++) {
17                 result.add(matrix[i][0]);
18         }
19             return result;
20         }
21         int xStart = 0;
22         int yStart = 0;
23         while (n > 0 && m > 0) {
24             if (m == 1 && n == 1) {
25                 result.add(matrix[yStart][xStart]);
26                 return result;
27             }
28             if (m == 1 && n > 1) {
29                 for (int i = 0; i < n; i++) {
30                     result.add(matrix[yStart][xStart + i]);
31                 }
32                 return result;
33             }
34             if (n == 1 && m > 1) {
35                 for (int i = 0; i < m; i++) {
36                     result.add(matrix[yStart + i][xStart]);
37                 }
38                 return result;
39             }
40             for (int i = 0; i < n - 1; i++) {
41                 result.add(matrix[yStart][xStart + i]);
42             }
43             for (int i = 0; i < m - 1; i++) {
44                 result.add(matrix[yStart + i][xStart + n - 1]);
45             }
46             for (int i = 0; i < n - 1; i++) {
47                 result.add(matrix[yStart + m - 1][xStart + n - 1 - i]);
48             }
49             for (int i = 0; i < m - 1; i++) {
50                 result.add(matrix[yStart + m - 1 - i][xStart]);
51             }
52             ++xStart;
53             ++yStart;
54             n = n - 2;
55             m = m - 2;
56         }
57         return result;
58     }
59 }