[题解]洛谷P2296 寻找道路

#include<cstdio>
#include<algorithm>
#include<cstring>
#include<iostream>
#include<queue>
using namespace std;
const int MAXN = 10010,MAXM = 200010,INF = 2147483647;
#define makenode(name,uu,dd); node name;name.u=(uu);name.d=(dd);

int N,M,S,T,first[MAXN],dis[MAXN];

struct edge{
    int u,v,next;
}e[MAXM];

inline int read(){
    int x=0,w=1;char c=getchar();
    while(c<‘0‘||c>‘9‘){
        if(c==‘-‘)w=-1;
        c=getchar(); 
    }
    while(c>=‘0‘&&c<=‘9‘){
        x=(x<<3)+(x<<1)+c-‘0‘;
        c=getchar();
    }
    return x*w;
}

int mm=0;
void insert(int u,int v){
    ++mm;e[mm].u=u;e[mm].v=v;e[mm].next=first[u];first[u]=mm;
}

int tcan[MAXN]={0},can[MAXN];
void bj(int n){
    tcan[n]=1;
    for(int i=first[n];i!=-1;i=e[i].next){
        int v=e[i].v;
        if(!tcan[v])bj(v);
    }
}

void start(){
    bj(T);
    memcpy(can,tcan,sizeof(tcan));
    for(int u=1;u<=N;u++)
        if(!tcan[u])
            for(int i=first[u];i!=-1;i=e[i].next)
                can[e[i].v]=0;
}

struct node{
    int u,d;
    bool operator <(const node &cmpp) const{
        return d>cmpp.d;
    }
};

void solve(){
    dis[T]=0;
    queue<int> q;
    q.push(T);
    while(!q.empty()){
        int no=q.front();
        q.pop();
        for(int i=first[no];i!=-1;i=e[i].next)
            if(can[e[i].v]){
                q.push(e[i].v);
                can[e[i].v]=0;
                dis[e[i].v]=dis[no]+1;
            }
    }
}

void print(){
    if(dis[S]==INF)printf("-1");
    else printf("%d",dis[S]);
}

int main(){
    memset(first,-1,sizeof(first));
    memset(dis,INF,sizeof(dis));
    N=read();M=read();
    for(int i=1;i<=M;i++){
        int u=read(),v=read();
        if(u!=v)insert(v,u);
    }
    S=read();T=read();
    start();
    solve();
    print();
    return 0;
}