Input: 3
Output:
[
 [ 1, 2, 3 ],
 [ 8, 9, 4 ],
 [ 7, 6, 5 ]
]

vector<vector<int>> generateMatrix(int n) {
    int num = 1;
    int top = 0, right = n - 1, bottom = n - 1, left = 0;

    vector<vector<int>> ret(n,vector<int>(n));
    while (num<=n*n)
    {
        //top
        for (int col = left; col <= right; col++) 
            ret[top][col] = num++;
        if (++top > bottom) break;

        //right
        for (int row = top; row <= bottom; row++) 
            ret[row][right] = num++;
        if (--right < left) break;

        //bottom
        for (int col = right; col >= left; col--) 
            ret[bottom][col] = num++;
        if (--bottom < top) break;

        //left
        for (int row = bottom; row >= top; row--) 
            ret[row][left] = num++;
        if (++left > right) break;//2.
    }
    return ret;
}