Follow up for N-Queens problem.

Now, instead outputting board configurations, return the total number of distinct solutions.

思路：

典型8皇后，回溯法，验证是否能放皇后位置，判断同一列，左上45°和135°方向。

bool isValid(vector
     
      & nQueens, int row, int col, int &n)     {         for (int i = 0; i < row;i++)         {             if (nQueens[i][col] == 'Q')return false;         }         for (int i = row - 1, j = col - 1; i >= 0&&j >= 0;i--,j--)         {             if (nQueens[i][j] == 'Q')return false;         }         for (int i = row - 1, j = col + 1; i >= 0&&j < n;i--,j++)         {             if (nQueens[i][j] == 'Q')return false;         }         return true;     }void totalNQueens(int& n, int row, vector
      
       & nQueens, int& total)     {         if (row == n)         {             total += 1;             return;         }         for (int col = 0; col < n;col++)         {             if (isValid(nQueens, row, col, n))             {                 nQueens[row][col] = 'Q';                 totalNQueens(n, row + 1, nQueens, total);                 nQueens[row][col] = '.';             }         }     }     int totalNQueens(int n)     {         vector
       
         nQueens(n, string(n, '.'));         int total = 0;         totalNQueens(n, 0, nQueens, total);         return total;     }