176. [USACO Feb07] 奶牛聚会

#include<iostream>
#include<cstdio>
#include<cstring>
#include<queue>
#define maxn 100010
using namespace std;
int n,m,T,num1,num2,head1[maxn],head2[maxn],dis1[maxn],dis2[maxn],f[maxn];
queue<int>q1;
queue<int>q2;
int ans[maxn],maxx;
struct node
{
    int u,v,t,pre;
}e1[maxn];
struct Node
{
    int u,v,t,pre;
}e2[maxn];
int init()
{
    int x=0;char s;s=getchar();
    while(s<'0'||s>'9')s=getchar();
    while(s>='0'&&s<='9'){x=x*10+s-'0';s=getchar();}
    return x;
}
void Add1(int from,int to,int dis)
{
    num1++;
    e1[num1].u=from;
    e1[num1].v=to;
    e1[num1].t=dis;
    e1[num1].pre=head1[from];
    head1[from]=num1;
}
void Add2(int from,int to,int dis)
{
    num2++;
    e2[num2].u=from;
    e2[num2].v=to;
    e2[num2].t=dis;
    e2[num2].pre=head2[from];
    head2[from]=num2;
}
void SPFA1()
{
    dis1[T]=0;
    q1.push(T);
    f[T]=1;
    while(!q1.empty())
      {
          int k=q1.front();
          q1.pop();
          f[k]=0;
          for(int i=head1[k];i;i=e1[i].pre)
            if(dis1[e1[i].v]>dis1[k]+e1[i].t)
              {
                dis1[e1[i].v]=dis1[k]+e1[i].t;
                if(f[e1[i].v]==0)
                  {
                    f[e1[i].v]=1;
                  q1.push(e1[i].v);    
                }
            }
      }
}
void SPFA2()
{
    dis2[T]=0;
    q2.push(T);
    f[T]=1;
    while(!q2.empty())
      {
          int k=q2.front();
          q2.pop();
          f[k]=0;
          for(int i=head2[k];i;i=e2[i].pre)
            if(dis2[e2[i].v]>dis2[k]+e2[i].t)
              {
                dis2[e2[i].v]=dis2[k]+e2[i].t;
                if(f[e2[i].v]==0)
                  {
                    f[e2[i].v]=1;
                  q2.push(e2[i].v);    
                }
            }
      }
}
int main()
{
    //freopen("sparty.in","r",stdin);
    //freopen("sparty.out","w",stdout);
    int x,y,z;
    n=init();m=init();T=init();
    for(int i=1;i<=m;i++)
      {
          x=init();y=init();z=init();
          Add2(x,y,z);Add1(y,x,z);
      }
    memset(dis1,127/3,sizeof(dis1));
    memset(dis2,127/3,sizeof(dis2));
    SPFA1();
    memset(f,0,sizeof(f));
    SPFA2();
    for(int i=1;i<=n;i++)
      ans[i]=dis1[i]+dis2[i];
    for(int i=1;i<=n;i++)
      maxx=max(maxx,ans[i]);
    printf("%d\n",maxx);
    return 0;
}