【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;
     }
 }