LeetCode-51-N-Queens

class Solution {
public:
    bool isValid(vector<string>& queen, int row,int col,int n){
        for(int i=0;i<row;++i){
            if(queen[i][col]=='Q')
                return false;
        }
        for(int i=row-1,j=col-1;i>=0&&j>=0;--i,--j){
            if(queen[i][j]=='Q')
                return false;
        }
        for(int i=row-1,j=col+1;i>=0&&j<n;--i,++j){
            if(queen[i][j]=='Q')
                return false;
        }
        return true;
    }
    void recurse(vector<string>& queen,vector<vector<string>>& res,int row,int n){
        if(row==n){
            res.push_back(queen);
            return;
        }
        for(int col=0;col!=n;++col){
            if(isValid(queen,row,col,n)){
                queen[row][col]='Q';
                recurse(queen,res,row+1,n);
                queen[row][col]='.';
            }
        }
    }
    vector<vector<string>> solveNQueens(int n) {
        vector<vector<string>> res;
        vector<string> queen(n,string(n,'.'));
        recurse(queen,res,0,n);
        return res;
    }
};