用邊建正反倆張圖
然後拍spfa計算兩張圖從x點出發的最短路。。
然後掃一遍計算答案
像我這樣把兩張圖連起來跑一次spfa也行

#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
#define ll long long
#define g getchar()
#define inf 0x3f3f3f3f
#define rep(i,n) for(int i=1;i<=n;++i)
using namespace std;
inline ll read(){
    ll x=0
 
,f=1;char ch=g;
    for(;ch<'0'||ch>'9';ch=g)if(ch=='-')f=-1;
    for(;ch>='0'&&ch<='9';ch=g)x=x*10+ch-'0';
    return x*f;
}
inline void out(ll x){
    int a[25],t=0;
    if(x<0)putchar('-'),x=-x;
    for(;x;x/=10)a[++t]=x%10;
    for(int i=t;i;--i)putchar('0'+a[i]);
    if(t==0)putchar
 
('0');
    putchar('\n');
}
struct re{int v,w,next;}ed[200005];
int dis[2005],pd[2005],dui[200005],head[2005],n,m,T,e,ans;
inline void ins(int x,int y,int w){ed[++e]=(re){y,w,head[x]};head[x]=e;}
void spfa(int S){
    memset(dis,inf,sizeof(dis));
    memset(pd,0,sizeof(pd));
    dis[S]=0;dui[1]=S;int tou=1,wei=1;
    for
 
(;tou<=wei;pd[dui[tou++]]=0){
        int u=dui[tou];
        for(int i=head[u];i;i=ed[i].next){
            int v=ed[i].v;
            if(dis[v]>dis[u]+ed[i].w){
                dis[v]=dis[u]+ed[i].w;
                if(!pd[v])dui[++wei]=v,pd[v]=1;
            }
        }
    }
}
int main(){
//  freopen("","r",stdin);
//  freopen("","w",stdout);
    n=read(),m=read(),T=read();
    rep(i,m){
        int x=read(),y=read(),w=read();
        ins(x,y,w);
        ins(y+n,x+n,w);
    }
    ins(T,T+n,0);
    spfa(T);
    rep(i,n){
        ans=max(dis[i]+dis[i+n],ans);
    }
    out(ans);
    return 0;
}