次短路LuoguP2865

Posted 2021-12-20 y15beta

#include<cstdio>
#include<cstring>
#include<queue>
using namespace std;

int n,r,head[1000005],dis1[1000005],dis2[1000005],vis[1000005],cnt;

struct edge
    int v,w,next;
e[1000005];

struct node
    int dis,u;
    bool operator<(const node &x)constreturn x.dis<dis;
;

inline void add(int u,int v,int w)
    e[++cnt].v=v;
    e[cnt].w=w;
    e[cnt].next=head[u];
    head[u]=cnt;


inline void dijkstra()
    priority_queue<node> q;
    q.push((node)0,1);
    dis1[1]=0;
    while(!q.empty())
        int x=q.top().u;
        int dis=q.top().dis;//必须有 
        q.pop();
        for(int i=head[x];i!=-1;i=e[i].next)
            int y=e[i].v;
            if(dis1[y]>=dis+e[i].w)
                dis2[y]=dis1[y];
                dis1[y]=dis+e[i].w;
                q.push((node)dis1[y],y);
            
            else if(dis2[y]>dis+e[i].w)
                dis2[y]=dis+e[i].w;
                q.push((node)dis2[y],y);//必须有 
            
        
    
//“必须有”因为不只考虑最优的

int main()
    memset(head,-1,sizeof(head));
    memset(dis1,0x3f,sizeof(dis1));
    memset(dis2,0x3f,sizeof(dis2));
    scanf("%d%d",&n,&r);
    for(int i=1;i<=r;i++)
        int a,b,c;
        scanf("%d%d%d",&a,&b,&c);
        add(a,b,c);
        add(b,a,c);
    
    dijkstra();
    printf("%d\n",dis2[n]);