http://acm.sjtu.edu.cn/OnlineJudge/problem/1233

回溯之后要记得改变状态啊啊啊啊啊啊啊！！！！

记住主要的思想是什么！！

#include <iostream>

using namespace std;
int M,cnt=0;
class graph
{
private:
    struct edgeNode{
        int end;
        edgeNode*next;
        int weight;
        edgeNode(int e,int w,edgeNode*n)
        {
            end= e;weight=w;next =n;
        }
    };
    struct verNode{
        int ver;
        edgeNode *head;
        verNode(edgeNode*h=NULL)
        {
            head = h;
        }
    };
    verNode*verList;
    int vSize,eSize;
public:
    graph(int size){
        vSize =size;
        eSize =0;
        verList = new verNode[size+1];
        for(int i=1;i<=size;++i)
            verList[i].ver = i;
    }
    void insert(int u,int v,int weight)
    {
        verList[u].head =new edgeNode(v,weight,verList[u].head);
        eSize++;
    }
    void dfs(int depth,int start,bool visit[])
    {
        visit[start] = true;
        if(depth==M){cnt++;return;}
        edgeNode* p=verList[start].head;
        while(p!=NULL)
        {
            if(visit[p->end]==false)
            {
                visit[p->end]=true;
                dfs(depth+1,p->end,visit);
                visit[p->end]=false;
            }
            p=p->next;
        }
        visit[start]=false;
    }
    void dfs(int depth,int start)
    {
        bool *visit=new bool[vSize+1];
        for(int i=1;i<=vSize;++i) visit[i]=false;

        dfs(depth,start,visit);
    }
};
int main()
{
    int n,m,u,v;
    cin>>n>>m>>start>>M;

    graph g(n);
    for(int i=0;i<m;++i)
    {
        cin>>u>>v;
        g.insert(u,v,1);
    }
    g.dfs(0,start);
    cout<<cnt;
    return 0;
}