BZOJ 1631 Cow Party

2*spfa.

#include<iostream>
#include<cstdio>
#include<cstring>
#include<queue>
#include<algorithm>
#define maxv 1050
#define maxe 200500
#define inf 2000000000
using namespace std;
queue <int> q;
int n,m,s,x,y,z,g[maxv],nume=0,dis[maxv][3];
struct edge
{
    int v,w,nxt;
}e[maxe];
bool vis[maxv];
void addedge(int u,int v,int w)
{
    e[++nume].v=v;
    e[nume].w=w;
    e[nume].nxt=g[u];
    g[u]=nume;
}
void spfa(int type)
{
    for (int i=1;i<=n;i++)
    {
        dis[i][type]=inf;
        vis[i]=false;
    }
    dis[s][type]=0;vis[s]=true;q.push(s);
    while (!q.empty())
    {
        int head=q.front();q.pop();
        for (int i=g[head];i;i=e[i].nxt)
        {
            int v=e[i].v;
             if ((i%2==type) && (dis[v][type]>dis[head][type]+e[i].w))
            {
                dis[v][type]=dis[head][type]+e[i].w;
                if (!vis[v]) {vis[v]=true;q.push(v);}
            }
        }
        vis[head]=false;
    }
}
int main()
{
    scanf("%d%d%d",&n,&m,&s);
    for (int i=1;i<=m;i++)
    {
        scanf("%d%d%d",&x,&y,&z);
        addedge(x,y,z);addedge(y,x,z);
    }
    spfa(0);
    spfa(1);
    int ans=0;
    for (int i=1;i<=n;i++)
        ans=max(ans,dis[i][0]+dis[i][1]);
    printf("%d\n",ans);
    return 0;
}