【leetcode刷题笔记】Spiral Matrix II

Given an integer n, generate a square matrix filled with elements from 1 to n2 in spiral order.

For example,
Given n = 3,

You should return the following matrix:

[
 [ 1, 2, 3 ],
 [ 8, 9, 4 ],
 [ 7, 6, 5 ]
]

---

题解：以前做过的Spiral Matrix是给一个矩阵螺旋式的输出，这道题是给一个n，螺旋式的生成矩阵。和第一个的做法类似，定义四个变量

left:当前未填的子矩阵的最左边界

right:当前未填的子矩阵的最右边界

up:当前未填子矩阵的最上边界

down:当前未填子矩阵的最下边界

然后每次进行四个循环，做一个螺旋。

代码如下：

public class Solution {
    public int[][] generateMatrix(int n) {
        int[][] matrix = new int[n][n];
        int left  = 0;
        int right = n-1;
        int up = 0;
        int down = n-1;
        int number = 0;
        while(number <= n*n-1){
            for(int i = left;i <= right;i++){
                matrix[up][i] = number+1;
                number++;
            }
            up++;
            
            for(int i = up;i <= down;i++){
                matrix[i][right] = number+1;
                number++;
            }
            right--;
            
            for(int i = right;i >= left;i--){
                matrix[down][i] = number+1;
                number++;
            }
            down--;
            
            for(int i = down;i >= up;i--){
                matrix[i][left] = number+1;
                number++;
            }
            left++;
        }
        return matrix;
    }
}