LeetCode 59. Spiral Matrix II

原题链接在这里：https://leetcode.com/problems/spiral-matrix-ii/

题目：

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：

从外圈到内圈螺旋着填数，如果n是奇数的话，最后正中间的数需要单独填写.

Time Complexity: O(n^2). Space: O(n^2).

AC Java:

class Solution {
    public int[][] generateMatrix(int n) {
        int [][] res = new int[n][n];
        int num = 1;
        int x = 0; 
        int y = 0;
        
        //由外到内，旋转加值
        while(n>1){
            for(int j = 0; j<n-1; j++){
                res[x][y++] = num++;
            }
            
            for(int i = 0; i<n-1; i++){
                res[x++][y] = num++;
            }
            
            for(int j = 0; j<n-1; j++){
                res[x][y--] = num++;
            }
            
            for(int i = 0; i<n-1; i++){
                res[x--][y] = num++;
            }
            
            n-=2;
            x++;
            y++;
        }
        
        if(n%2 == 1){
            res[x][y] = num;
        }
        
        return res;
    }
}

类似Spiral Matrix. 

跟上Spiral Matrix III.