Question

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：定义四个边界。注意：退出应该在每次移动后判定。

import java.util.ArrayList;
public class Solution {
  public ArrayList<Integer> printMatrix(int [][] matrix) {
    if(matrix.length == 0 || matrix[0].length == 0) return null;
    int top = 0;
    int down = matrix.length-1;
    int left = 0;
    int right = matrix[0].length-1;
    ArrayList<Integer> result = new ArrayList<>();
    while(true){
      //向右移动
      for(int i = left; i <= right; i++){
        result.add(matrix[top][i]);
      }
      top++;
      if(top>down) break;
      //向下移动
      for(int i = top; i <= down; i++){
        result.add(matrix[i][right]);
      }
      right--;
      if(left>right) break;
      //向左移动
      for(int i = right; i>= left; i--){
        result.add(matrix[down][i]);
      }
      down--;
      if(top>down) break;
      //向上移动
      for(int i = down; i>= top; i--){
        result.add(matrix[i][left]);
      }
      left++;
      if(left>right) break;
    }
    return result;
  }
}