最短路 CF954D Fight Against Traffic

CF954D Fight Against Traffic

被水题成功报复。。。

code：

#include <iostream>

#include <cstdio>

#include <queue>

#include <cstring>

using namespace std;

inline int read(){

	int sum=0,f=1; char ch=getchar();

	while(ch<'0'||ch>'9'){if(ch=='-')f=-1; ch=getchar();}

	while(ch>='0'&&ch<='9'){sum=(sum<<1)+(sum<<3)+ch-'0'; ch=getchar();}

	return sum*f;

}

const int wx=2017;

int head[wx],dis1[wx],dis2[wx];

int vis[wx],fl[wx][wx];

int num,n,m,s,t,ans;

struct e{

	int nxt,to,dis;

}edge[wx*2];

void add(int from,int to,int dis){

	edge[++num].nxt=head[from];

	edge[num].to=to;

	edge[num].dis=dis;

	head[from]=num;

}

struct node{

	int u,d;

	friend bool operator < (const node & a,const node & b){

		return a.d>b.d;

	}

};

priority_queue<node > q;

void Dij(int S,int dis[]){

	for(int i=1;i<=n;i++)dis[i]=0x3f3f3f3f,vis[i]=0;

	dis[S]=0; q.push((node){S,0});

	while(q.size()){

		int u=q.top().u; q.pop();

		if(vis[u])continue; vis[u]=1;

		for(int i=head[u];i;i=edge[i].nxt){

			int v=edge[i].to;

			if(dis[v]>dis[u]+edge[i].dis){

				dis[v]=dis[u]+edge[i].dis;

				q.push((node){v,dis[v]});

			}

		}

	}

}

int main(){

	n=read(); m=read();

	s=read(); t=read();

	for(int i=1;i<=m;i++){

		int x,y;

		x=read(); y=read();

		add(x,y,1); add(y,x,1);

		fl[x][y]=1; fl[y][x]=1;

	}

	Dij(s,dis1); Dij(t,dis2);

	for(int i=1;i<=n;i++){

		for(int j=i+1;j<=n;j++){

			if(fl[i][j])continue;

			int tmp1=dis1[i]+dis2[j]+1;

			int tmp2=dis1[j]+dis2[i]+1;

			if(min(tmp1,tmp2)>=dis1[t])ans++;

		}

	}

	printf("%d\n",ans);

	return 0;

}