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 ]
]
You should return [1,2,3,6,9,8,7,4,5].

public class Solution {
    public List<Integer> spiralOrder(int[][] matrix) {
        ArrayList<Integer> res = new ArrayList<Integer>();
        if (matrix==null || matrix.length==0 || matrix[0].length==0) return res;
        int Min = Math.min(matrix.length, matrix[0].length);
        int levelMax = Min / 2;
        for (int level=0; level<levelMax; level++) {
            for (int i=level; i<matrix[0].length-1-level; i++) {
                res.add(matrix[level][i]);
            }
            for (int i=level; i<matrix.length-1-level; i++) {
                res.add(matrix[i][matrix[0].length-1-level]);
            }
            for (int i=matrix[0].length-1-level; i>level; i--) {
                res.add(matrix[matrix.length-1-level][i]);
            }
            for (int i=matrix.length-1-level; i>level; i--) {
                res.add(matrix[i][level]);
            }
        }
        if (Min % 2 == 1) {
            if (matrix.length < matrix[0].length) {
                for (int i=levelMax; i<matrix[0].length-levelMax; i++) {
                    res.add(matrix[levelMax][i]);
                }
            }
            else {
                for (int i=levelMax; i<matrix.length-levelMax; i++) {
                    res.add(matrix[i][levelMax]);
                }
            }
        }
        return res;
    }
}