The n-queens puzzle is the problem of placing n queens on an n×n chessboard such that no two queens attack each other.

Given an integer n, return all distinct solutions to the n-queens puzzle.

Each solution contains a distinct board configuration of the n-queens’ placement, where ‘Q’ and ‘.’ both indicate a queen and an empty space respectively.

class Solution {
public:
    /**
     * Get all distinct N-Queen solutions
     * @param n: The number of queens
     * @return: All distinct solutions
     * For example, A string '...Q' shows a queen on forth position
     */
    vector<vector<string> > res;

    void dfs(vector<int> &solution,int n,int cur){
        if(cur==n){
            string str="";
            for(int i=0;i<n;i++){
                str+='.';
            }
            vector<string> vec;
            for(int i=0;i<n;i++){
                string tmp=str;
                tmp[solution[i]]='Q';
                vec.push_back(tmp);
            }
            res.push_back(vec);
            return;
        }

        for(int i=0;i<n;i++){
            solution.push_back(i);
            bool ok=true;
            for(int j=0;j<cur;j++){
                if(i==solution[j]|| (cur+i)==(j+solution[j])|| (cur-i)==(j-solution[j])){
                    ok=false;
                    break;
                }
            }
            if(ok){
                dfs(solution,n,cur+1);
                solution.pop_back();
            }else{
                solution.pop_back();
            }
        }


    }

    vector<vector<string> > solveNQueens(int n) {
        // write your code here
        vector<int> solution;
        dfs(solution,n,0);

        return res;
    }
};