深度优先搜索：简单讲就是搜到某条路尽头，再掉头回溯搜其他的路。此中重点是尽头的判断，和对走过的路进行标记。

一般采用递归的写法，模板大致如下：

DFS(node,visited):
    if node is in visited:return
    add node to visited;
    主干操作;

    for each neighbor in node.neighbors:
        if neighbor is not in visited(能走是邻居且未访问过): DFS(neibor,visited)
        
    回溯，把当前node影响消除，同时视情况将vis[当前node]修改为0

P1. 洛谷P1219八皇后为例

#include<iostream>
#include<cstring>
#include<vector>
using namespace std;
int n=0;
vector<int> ans;
int nums=0;

void showcurboard(bool** boa)
{
    for(int i=1;i<=n;i++)
    {
        for(int j=1;j<=n;j++)
        cout<<boa[i][j];
        cout<<endl;
    }
}

void countans(vector<int> a)
{
    for(int i=0;i<a.size();i++) cout<<a[i]<<" ";
    cout<<endl;
}

bool inthemap(int a,int b)
{
    return 1<=a&&a<=n&&1<=b&&b<=n;
}

void dealqueen(bool** boa,int a,int b,bool putortake)
{
    for(int i=0;i<n+2;i++)
    {
        if(inthemap(a,i)) boa[a][i]=putortake;
        if(inthemap(i,b)) boa[i][b]=putortake;
    }
    for(int i=-n;i<=n;i++)
    {
        if(inthemap(a+i,b+i)) boa[a+i][b+i]=putortake;
        if(inthemap(a+i,b-i)) boa[a+i][b-i]=putortake;
    }
}

void DFS(bool** &boa,int line,int column,bool** vis)//一定是node,visited
{
    if(vis[line][column]==1) return;//主要是初始引入时判断，line==1时判断，其它节点下下面for那都判断过了
    vis[line][column]=1;
    ans.push_back(column);
    dealqueen(boa,line,column,0);
    if(line==n)
    {
        nums++;
        if(nums<=3) countans(ans);
    }
    
    for(int j=1;j<=n;j++)
    {
        if(vis[line+1][j]==0&&boa[line+1][j]==1&&line+1<n+1) DFS(boa,line+1,j,vis);
    }

    vis[line][column]=0;//下四条都是回溯消除痕迹
    dealqueen(boa,line,column,1);
    ans.pop_back();
    for(int i=0;i<ans.size();i++) dealqueen(boa,i+1,ans[i],0);
}

int main()
{
    cin>>n;
    bool** board=new bool*[n+2];
    bool** visited=new bool*[n+2];
    for(int i=0;i<n+2;i++)
    {
        board[i]=new bool[n+2];
        visited[i]=new bool[n+2];
        memset(board[i],1,sizeof(board[i]));
        memset(visited[i],0,sizeof(visited[i]));
    }
    //showcurboard(board);
    //dealqueen(board,2,3,0); //showcurboard(board);
    for(int j=1;j<=n;j++)
    DFS(board,1,j,visited);
    cout<<nums;
    return 0;
}