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.

 import java.util.ArrayList;

 public class Solution {

     public ArrayList<Integer> printMatrix(int [][] matrix) {

         ArrayList<Integer> arr = new ArrayList<Integer>();

         int up = 0;

         int left = 0;

         int down = matrix.length - 1;

         int right = matrix[0].length - 1;

         while(left<=right && up<=down){

             for(int i = left ; i <= right ; i++)

                 arr.add(matrix[up][i]);

             up++;

             for(int i = up ; i <= down ; i++)

                 arr.add(matrix[i][right]);

             right--;

             if(up > down) break;

             for(int i = right ; i >= left ; i--)

                 arr.add(matrix[down][i]);

             down--;

             if(left > right) break;

             for(int i = down ; i >= up ; i--)

                 arr.add(matrix[i][left]);

             left++;

         }

         return arr;

     }

 }