[LeetCode] Spiral Matrix II

Problem:

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 ]
]

Solution:O(n^2)

public class Solution {
    public int[][] generateMatrix(int n) {
        if(n<0) return null;
        int[][] ret = new int[n][n];
        int start=1;

        int x=0;

        int y=0;
        for(int i=n;i>0;i-=2){
            if(i==1){
                ret[x][y]=start;
            }else{
                for(int j=0;j<i-1;j++){
                    ret[x][y++]=start++;   
                }
                for(int j=0;j<i-1;j++){
                    ret[x++][y]=start++;
                }
                for(int j=0;j<i-1;j++){
                    ret[x][y--]=start++;
                }
                for(int j=0;j<i-1;j++){
                    ret[x--][y]=start++;
                }
                x++;
                y++;
            }
        }
        return ret;
    }
}