剑指offer-顺时针打印矩阵

输入一个矩阵，按照从外向里以顺时针的顺序依次打印出每一个数字，例如，如果输入如下4 X 4矩阵：
1 2 3 4
5 6 7 8
9 10 11 12
13 14 15 16
则依次打印出数字
1,2,3,4,
8,12,16,15,
14,13,9,5,
6,7,11,10.

思路
使用up存储最上边行号，down存最下边行号，left存最左边列号，right存最右边列号
将上方的值添加进list中，通过判断up>down结束，再将右边的值放入...

Java

import java.util.ArrayList;
public class Solution {
  public ArrayList<Integer> printMatrix(int [][] matrix) {
    ArrayList<Integer> list = new ArrayList<>();
    if(matrix == null || matrix.length == 0 || matrix[0].length == 0) return list;
    
    int up = 0;
    int down = matrix.length-1;
    int left = 0;
    int right = matrix[0].length-1;
    
    while (true) {
      for (int col = left; col <= right; col++)
        list.add(matrix[up][col]);
      up++;
      if (up > down)  break;
      for (int row = up; row <= down; row++)
        list.add(matrix[row][right]);
      right--;
      if (right < left)  break;
      for (int col2 = right; col2 >= left; col2--)
        list.add(matrix[down][col2]);
      down--;
      if (up > down)  break;
      for (int row2 = down; row2 >= up; row2--)
        list.add(matrix[row2][left]);
      left++;
      if (right < left)  break;
    }
    return list;
  }
}