题解 | #螺旋矩阵#定义4个边界，然后顺水推舟，推荐！

import java.util.ArrayList;
public class Solution {
    public ArrayList<Integer> spiralOrder(int[][] matrix) {
        //思路 定义左右上下边界 边界不重合则一直循环
        ArrayList<Integer> list = new ArrayList<>();
        if(matrix == null || matrix.length == 0){
            return list;
        }
        int left = 0;
        int right = matrix[0].length - 1;
        int up = 0;
        int down = matrix.length - 1;
        while(left <= right && up <= down){
            for(int i = left;i<=right;i++){
                list.add(matrix[up][i]);
            }
            up ++;
            if(up > down){
                break;
            }
            for(int i = up;i<=down;i++){
                list.add(matrix[i][right]);
            }
            right --;
            if(left > right){
                break;
            }
            for(int i = right;i>=left;i--){
                list.add(matrix[down][i]);
            }
            down --;
            if(up > down){
                break;
            }
            for(int i = down;i>=up;i--){
                list.add(matrix[i][left]);
            }
            left ++;
            if(left > right){
                break;
            }
        }
        return list;
    }
}