题解 | #顺时针打印矩阵#

import java.util.*;
import java.util.ArrayList;
public class Solution {
    public ArrayList<Integer> printMatrix(int [][] matrix) {
       ArrayList<Integer> rest = new ArrayList<>();
       if(matrix.length == 0){
            return rest;
       }
       //上左边界
       int up = 0;
       int left = 0;
       //下右边界
       int down = matrix.length -1;
       int right = matrix[0].length -1;
        while(left <= right && up <= down){
            //上边界从左到右遍历
            for(int i = left;i <= right;i++){
                rest.add(matrix[up][i]);
            }
            //上边界向下
            up++;
            if(up > down){
                break;
            }
            //右边界从上到下遍历
            for(int i = up;i <= down;i++){
                rest.add(matrix[i][right]);
            }
            //右边界向左
            right--;
            if(left > right){
                break;
            }
            //下边界从右到左遍历
            for(int i = right;i >= left;i--){
                rest.add(matrix[down][i]);
            }
            //下边界向上
            down--;
            if(up > down){
                break;
            }
            //左边界从下到上遍历
            for(int i = down;i >= up;i--){
                rest.add(matrix[i][left]);
            }
            //左边界向右
            left++;
            if(left > right){
                break;
            }
        }
       return rest;
    }
}