class Solution {public:    vector<vector<int> > generateMatrix(int n) {        //n by n matrix        vector<vector<int> > matrix(n, vector<int>(n,0));        if(n == 0)            return matrix;        int level = (n%2==0)?(n/2):(n/2+1);        int number = 1;        for(int i = 0; i < level; i ++)        {            //from up-left to up-right            for(int j = i; j < n-i; j ++)                matrix[i][j] = number++;            //from up-right to down-right            for(int j = i+1; j < n-i; j ++)                matrix[j][n-1-i] = number++;            //from down-right to down-left            for(int j = n-1-i-1; j >= i; j --)                matrix[n-1-i][j] = number++;            //from down-left to up-left            for(int j = n-1-i-1; j > i; j --)                matrix[j][i] = number++;        }        return matrix;    }};