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=10010;
const int MOD = 1e9+7;
#define LL long long
 
void fre() {
    freopen("in.txt","r",stdin);
}
inline int r() {
    int x=0,f=1;char ch=getchar();
    while(ch>'9'||ch<'0') {if(ch=='-') f=-1;ch=getchar();}
    while(ch>='0'&&ch<='9') { x=x*10+ch-'0';ch=getchar();}return x*f;
}
 
vector<int>g[101]; 
LL countt[110];
LL val[110]={0};
 
int main(){
    int n,m,s,t;
    n=r();
    m=r();
    s=r();
    t=r();
    for(int i=0;i<=n;i++)
        g[i].clear();
    for(int i=0;i<m;i++){
        int x,y;
        x=r();
        y=r();
        g[x].push_back(y);
        g[y].push_back(x);
    }
    val[s]=1;
    for(int i=1;i<=t;i++){
        for(int j=0;j<n;j++)
            countt[j]=0;
        for(int j=0;j<n;j++){
            LL num=val[j];
            for(int p=0;p<(int)g[j].size();p++){
                int v=g[j][p];
                countt[v]+=num;
            }
        }
        for(int j=0;j<n;j++){
            val[j]=countt[j];
        }
    }
    LL ans=0;
    for(int i=0;i<n;i++)
        ans+=val[i];
    printf("%lld
",ans);
    return 0;
}