【回溯】51. N-Queens

在52. N-Queens II的基础上微改代码即可，添加棋盘的输出：

class Solution {
public:
    vector<vector<string>> solveNQueens(int n) {
        vector<int> temp;
        vector<vector<string>> res;
        vector<string> nullpoint;
        init(nullpoint,n);
        find(res,temp,0,n,nullpoint);
        return res;
    }
     void init(vector<string>& nullpoint,int total)  //初始化一个空的棋盘格，各位置均为字符'.'
     {
         string s;
         for(int i = 0;i<total;i++)
             s= s+'.';
         for(int i=0;i<total;i++)
             nullpoint.push_back(s);
     }
     bool check(vector<int>& temp, int row, int col)
    {
        for (int i = 0; i < temp.size(); i++)
        {
            if (row == i || col == temp[i] || abs((row - i)) == abs((col - temp[i])))
                return 0;
        }  
        return 1;
    }
    void find(vector<vector<string>>& res, vector<int>& temp, int row, int total,vector<string> nullpoint)
    {
        if (row == total)
        {
            for(int i=0;i<temp.size();i++)
                nullpoint[i][temp[i]]='Q';    //得到一解后，将nullpoint中的(i,temp[i])位置替换为'Q'
            res.push_back(nullpoint);            
            return;
        }
        int i = row;
        for (int j = 0; j < total; j++)
        {
            if (check(temp,i,j))
            {
                temp.push_back(j);
                find(res, temp, i + 1, total,nullpoint); 
                temp.pop_back();
            }
        }
    }
    
};