【leetcode】Spiral Matrix II

Given an integer n, generate a square matrix filled with elements from 1 to n^2 in spiral order.

For example,
Given n = 3,

You should return the following matrix:
[
[ 1, 2, 3 ],
[ 8, 9, 4 ],
[ 7, 6, 5 ]
]

 

class Solution {
public:
    vector<vector<int> >generateMatrix(int n) {
        int m=0;
        vector<vector<int> >a;
        a.resize(n,vector<int>(n));
        if(n==0)
            return a;
        int x=0;
        int y=0;
        m=a[0][0]=1;

        while(m<n*n)
        {
            while(y<n-1&&!a[x][y+1])
            {
                m++;
                y++;
                a[x][y]=m;
            }
            while(x<n-1&&!a[x+1][y])
            {
                m++;
                x++;
                a[x][y]=m;
            }
           
            while(y-1>=0&&!a[x][y-1])
            {
                m++;
                y--;
                a[x][y]=m;
            }

            while(x-1>=0&&!a[x-1][y])
            {
                m++;
                x--;
                a[x][y]=m;
            }
        }   

        return a;
    }
};