East Central North America Region 2015

E

每过一秒，当前点会把它的值传递给所有相邻点，问t时刻该图的值

 #include <iostream>
 #include <cstdio>
 #include <algorithm>
 #include <list>
 #include <map>
 #include <stack>
 #include <vector>
 #include <cstring>
 #include <sstream>
 #include <string>
 #include <cmath>
 #include <queue>
 using namespace std;
 #define clc(a,b) memset(a,b,sizeof(a))
 #define inf 0x3f3f3f3f
 const int N=;
 const int MOD = 1e9+;
 #define LL long long

 void fre() {
     freopen("in.txt","r",stdin);
 }
 inline int r() {
     int x=,f=;char ch=getchar();
     while(ch>''||ch<'') {if(ch=='-') f=-;ch=getchar();}
     while(ch>=''&&ch<='') { x=x*+ch-'';ch=getchar();}return x*f;
 }

 vector<int>g[];
 LL countt[];
 LL val[]={};

 int main(){
     int n,m,s,t;
     n=r();
     m=r();
     s=r();
     t=r();
     for(int i=;i<=n;i++)
         g[i].clear();
     for(int i=;i<m;i++){
         int x,y;
         x=r();
         y=r();
         g[x].push_back(y);
         g[y].push_back(x);
     }
     val[s]=;
     for(int i=;i<=t;i++){
         for(int j=;j<n;j++)
             countt[j]=;
         for(int j=;j<n;j++){
             LL num=val[j];
             for(int p=;p<(int)g[j].size();p++){
                 int v=g[j][p];
                 countt[v]+=num;
             }
         }
         for(int j=;j<n;j++){
             val[j]=countt[j];
         }
     }
     LL ans=;
     for(int i=;i<n;i++)
         ans+=val[i];
     printf("%lld\n",ans);
     return ;
 }