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 ] ]

class  Solution {
public:
    vector<vector< int> > generateMatrix( int n) 
    {
       vector<vector< int>>  result;
        if(n== 0)  return result;
       
        for( int i= 0;i<n;i++)
       {
           vector< int> v;
            for( int j= 0;j<n;j++)
            v.push_back( 0);
            result.push_back(v);
        }
         int len=n- 1;
         int num= 1;
         int x= 0;
        result[ 0][ 0]= 1;
         while(len>= 1)
        {
             for( int i= 0;i<len;i++)
                result[x][x+i]=num++;
             for( int i= 0;i<len;i++)
                result[x+i][x+len]=num++;
             for( int i= 0;i<len;i++)
                result[x+len][x+len-i]=num++;
             for( int i= 0;i<len;i++)
                result[x+len-i][x]=num++;
            len=len- 2;
            x++;
        }
         if(len== 0) result[x][x]=num;
         return result;
    }
};