思路：正向建边，一遍Dijkstra，反向建边，再一遍Dijkstra。ans加在一起输出最大值。
（SPFA也行……）

// by SiriusRen
#include <queue>
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
#define N 1005
int n,m,X,tot=0,maxx=0,first[N],v[N*N],w[N*N],next[N*N],xx[N*N],yy[N*N],zz[N*N],d[N],ans[N];
struct node{int weight,now;}jy;
void add(int x,int y,int z){w[tot]=z,v[tot]=y,next[tot]=first[x],first[x]=tot++;}
bool operator < (node a,node b){return a.weight>b.weight;}
void Dijkstra(int x){
    priority_queue<node>pq;
    bool vis[N];memset(vis,0,sizeof(vis)),memset(d,0x3f,sizeof(d));
    jy.weight=0,jy.now=x,d[x]=0;
    pq.push(jy);
    while(!pq.empty()){
        node temp=pq.top();pq.pop();
        if(vis[temp.now])continue;
        vis[temp.now]=1;
        for(int i=first[temp.now];~i;i=next[i])
            if(!vis[v[i]]&&d[v[i]]>d[temp.now]+w[i]){
                d[v[i]]=d[temp.now]+w[i];
                jy.now=v[i];
                jy.weight=temp.weight+w[i];
                pq.push(jy);
            }
    }
    for(int i=1;i<=n;i++)ans[i]+=d[i];
}
void spfa(int x){
    bool vis[N];memset(vis,0,sizeof(vis)),memset(d,0x3f,sizeof(d));
    queue<int>q;
    vis[x]=1,d[x]=0,q.push(x);
    while(!q.empty()){
        int temp=q.front();q.pop(),vis[temp]=0;
        for(int i=first[temp];~i;i=next[i])
            if(d[temp]+w[i]<d[v[i]]){
                d[v[i]]=d[temp]+w[i];
                if(!vis[v[i]])q.push(v[i]),vis[v[i]]=1;
            }
    }
    for(int i=1;i<=n;i++)ans[i]+=d[i];
}
int main()
{
    scanf("%d%d%d",&n,&m,&X);
    memset(first,-1,sizeof(first));
    for(int i=1;i<=m;i++)scanf("%d%d%d",&xx[i],&yy[i],&zz[i]),add(xx[i],yy[i],zz[i]);
    Dijkstra(X);
    memset(first,-1,sizeof(first)),tot=0;
    for(int i=1;i<=m;i++)add(yy[i],xx[i],zz[i]);
    Dijkstra(X);
    for(int i=1;i<=n;i++)maxx=max(maxx,ans[i]);
    printf("%d",maxx);
}

POJ 3268 Dijkstra+priority_queue或SPFA_#define