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 ]
]

    vector<vector<int> > generateMatrix(int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<vector<int> > matrix;
        if(n<=0)
            return matrix;
        int i,j;
        for(i=0;i<n;i++)
            matrix.push_back(vector<int>(n));
        
        int start = 0;
        int cnt = 1;
        while(n>2*start)
        {
            int end = n-1-start;
            for(i=start;i<=end;i++)
                matrix[start][i] = cnt++;
            for(i=start+1;i<=end;i++)
                matrix[i][end] = cnt++;
            for(i=end-1;i>=start;i--)
                matrix[end][i] = cnt++;
            for(i=end-1;i>start;i--)
                matrix[i][start] = cnt++;
            ++start;
        }
        return matrix;
        
        
    }