LeetCode 54. Spiral Matrix--给定一个n*m的二位数组，顺时针打印出元素，如果没有元素，打印出[]

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

Example 1:

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

Example 2:

Input:
[
  [1, 2, 3, 4],
  [5, 6, 7, 8],
  [9,10,11,12]
]
Output: [1,2,3,4,8,12,11,10,9,5,6,7]

import java.util.ArrayList;
import java.util.List;

/**
 * @Desc
 * @Author liuyazhou
 * @CreateTime 2018/11/18 15:40
 **/
public class LeetCode_54_SpiralMatrix {


   public static void main(String[] args) {
        int[][] mat