class Solution { public:     vector<vector<int> > generateMatrix(int n) {         vector<vector<int> > matrix(n,vector<int>(n));         int count = 1;         int row_start = 0,row_end = n-1,col_start = 0,col_end = n-1;                   while(count <= n*n)         {            for(int i=col_start;i<=col_end;i++)                 matrix[row_start][i] = count++;             row_start++;                           for(int i=row_start;i<=row_end;i++)                 matrix[i][col_end] = count++;             col_end--;                           for(int i=col_end;i>=col_start;i--)                 matrix[row_end][i] = count++;             row_end--;                           for(int i=row_end;i>=row_start;i--)                 matrix[i][col_start] = count++;             col_start++;                 }         return matrix;     } };