顺时针打印矩阵（含思路解答示意图）【剑指offer——JAVA实现】

【考点：规律】顺时针打印矩阵

题目描述

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.

解题思路

如下图所示

设置四个点，分别对应矩形的四个角，然后按照顺时针的顺序输出每一个圈的四条边，循环终止条件是左上角的点坐标不超过矩形中点坐标。

代码提交

import java.util.ArrayList;
public class Solution {
    public ArrayList<Integer> printMatrix(int [][] matrix) {
        ArrayList<Integer> list = new ArrayList<Integer>();
        if(matrix == null || matrix.length == 0) return list;
        
        int rowNum = matrix.length;
        int colNum = matrix[0].length;
        
        Point leftTopPoint = new Point(0, 0);
        Point rightTopPoint = new Point(colNum - 1, 0);
        Point leftDownPoint = new Point(0, rowNum - 1);
        Point rightDownPoint = new Point(colNum - 1, rowNum - 1);
        //循环终止条件
        while(leftTopPoint.x * 2 < colNum && leftTopPoint.y * 2 < rowNum) {
            //绘制第一行
            for(int i = leftTopPoint.x; i <= rightTopPoint.x; i++)
                list.add(matrix[leftTopPoint.y][i]);
            //绘制第一列
            for(int j = rightTopPoint.y + 1; j <= rightDownPoint.y; j++)
                list.add(matrix[j][rightTopPoint.x]);
            //绘制第二行
            for(int k = rightDownPoint.x - 1; k >= leftDownPoint.x; k--)
                if(rightDownPoint.y > rightTopPoint.y)//还要检查第二行的纵坐标大于第一行
                    list.add(matrix[rightDownPoint.y][k]);
            //绘制第二列
            for(int m = leftDownPoint.y - 1; m > leftTopPoint.y; m--)
                if(leftDownPoint.x < rightDownPoint.x)//还要检查第二列横坐标小于第一列
                    list.add(matrix[m][leftDownPoint.x]);
            //左上角
            leftTopPoint.x++;
            leftTopPoint.y++;
            //右上角
            rightTopPoint.x--;
            rightTopPoint.y++;
            //右下角
            rightDownPoint.x--;
            rightDownPoint.y--;
            //左下角
            leftDownPoint.x++;
            leftDownPoint.y--;
        }
        return list;
    }
    
    
    private class Point{
        int x;
        int y;
        
        Point(int x, int y) {
            this.x = x;
            this.y = y;
        }
    }
}

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