记录编号	139500	评测结果	AAAAAAAAAA
题目名称	[NOIP 2014]寻找道路	最终得分	100
用户昵称	Asm.Def	是否通过	通过
代码语言	C++	运行时间	0.098 s
提交时间	2014-11-12 16:35:26	内存使用	0.58 MiB

/*======================================================*/
/*========================ASM.DEF=======================*/
/*======================================================*/
#include <cstdio>
#include <cctype>
#include <iostream>
#include <cmath>
#include <cstring>
#include <algorithm>
#include <vector>
#include <queue>
#include <deque>
#include <list>
using namespace std;
#if defined WJX_Asm_Def_DEBUG
FILE *in = fopen("test", "r");
#define out stdout
#else
FILE *in = fopen("roadb.in", "r");
FILE *out = fopen("roadb.out", "w");
#endif
inline bool getd(int &x){
	int c = fgetc(in);
	bool minus = 0;
	while(c != '-' && c != EOF && !isdigit(c))c = fgetc(in);
	if(c == EOF)return 0;
	if(c == '-'){
		x = 0;
		minus = 1;
	}
	else x = c - '0';
	while(isdigit(c = fgetc(in)))x = x * 10 - '0' + c;
	return 1;
}
#define iter(v) v::iterator
#define pb push_back
#define pf push_front
#define ppf pop_front
/*===========================================================*/
const int maxn = 10000 + 5, INF = 0x3f3f3f3f;
list<int> adj[maxn];
vector<int> back[maxn];
int n, m, s, t;
int dis[maxn];
bool in_p[maxn] = {0}, known[maxn] = {0}, linked[maxn] = {0};

void dfs(int cur){//判断是否与t联通
	iter(vector<int>) it;
	for(it = back[cur].begin();it !=back[cur].end();++it){
		if(!known[*it]){
			linked[*it] = known[*it] = 1;
			dfs(*it);
		}
	}
}

inline void init(){
	getd(n), getd(m);
	int x, y;
	while(m--){
		getd(x), getd(y);
		if(x == y)continue;
		adj[x].pb(y);
		back[y].pb(x);
	}
	getd(s), getd(t);
	linked[t] = 1, known[t] = 1, in_p[t] = 1;
	dfs(t);
	iter(list<int>) it;
	for(int i = 1;i <= n;++i){
		bool ok = 1;
		for(it = adj[i].begin();it != adj[i].end();++it)
			if(!linked[*it]){ok = 0;break;}
		if(ok)in_p[i] = 1;
		dis[i] = INF;
	}
}
deque<int> Q;
bool inQ[maxn] = {0};
inline void work(){
	if(!in_p[s]){fprintf(out, "-1\n");return;}
	Q.pb(s), inQ[s] = 1, dis[s] = 0;
	int k;
	iter(list<int>) it;
	while(!Q.empty()){
		k = Q.front();inQ[k] = 0;Q.ppf();
		for(it = adj[k].begin();it != adj[k].end();++it){
			if(!in_p[*it]){
				it = adj[k].erase(it);
				--it;
			}
			else if(dis[k] + 1 < dis[*it]){
				dis[*it] = dis[k] + 1;
				if(inQ[*it])continue;
				inQ[*it] = 1;
				if(Q.empty() || dis[*it] <= dis[Q.front()])Q.pf(*it);//Small Label First
				else Q.pb(*it);
			}
		}
	}
	if(dis[t] == INF)fprintf(out, "-1\n");
	else fprintf(out, "%d\n", dis[t]);
}

int main(){
	init();
	
	work();
	
	#if defined WJX_Asm_Def_DEBUG
	cout << (double)clock() / CLOCKS_PER_SEC << endl;
	#endif
	return 0;
}