#include<iostream>#include<vector>#include<cmath>using namespace std;class Solution{public:    int res=0;    int totalNQueens(int n)    {        vector<int> state(n,-1);        helper(n,0,state);        return res;    }    void helper(int n,int start,vector<int> &state)    {        if(start==n)        {            res++;            return;        }        int i;        for(i=0; i<n; i++)        {            if(isValid(state,start,i))            {                state[start]=i;                helper(n,start+1,state);                state[start]=-1;            }        }    }    bool isValid(vector<int> &state,int row,int col)    {        int i;        for(i=0; i<row; i++)            if(state[i]==col||row-i==abs(state[i]-col))                return false;        return true;    }};int main(){    Solution s;    cout<<s.totalNQueens(4)<<endl;}