题解 | #顺时针打印矩阵#

一开始怎么也想不明白为啥要再每个for循环下面加if判断，不加的话这个例子[[1,2,3,4]]会输出1234321。

import java.util.*;
import java.util.ArrayList;
public class Solution {
    public static ArrayList<Integer> printMatrix(int[][] matrix) {
        ArrayList<Integer> res = new ArrayList();
        if (matrix.length == 0) return null;
        int up = 0;
        int down = matrix.length - 1;
        int left = 0;
        int right = matrix[0].length - 1;
        while (left <= right && up <= down) {
            for (int i = left; i <= right; i++){
                res.add(matrix[up][i]);
            }
            up++;
            if(up > down)break;
            for (int i = up; i <=down; i++){
                res.add(matrix[i][right]);
            }
            right--;
            if(left > right)break;
            for (int i = right; i >=left;i--){
                res.add(matrix[down][i]);
            }
            down--;
            if(up > down)break;
            for (int i = down; i >=up; i--){
                res.add(matrix[i][left]);
            }
            left++;
            if(left > right)break;
        }
        return res;
    }
}