次短路 & dijkstra

次短路 & dijkstra——P2865 [USACO06NOV] Roadblocks G

#include<bits/stdc++.h>
using namespace std;
#define IOS ios::sync_with_stdio(0);cout.tie(0);cin.tie(0);
#define endl '\n'
#define int long long
typedef pair<int,int> PII;
const int N=5141;
const int M=1e5+1145;
const int INF=0x3f3f3f3f3f3f3f3f;
struct Edge{
	int to,val;
};
struct Node{
	int dis,pos;
	bool operator >(const Node& other)const{
		return dis>other.dis;
	}
};
vector<Edge>edge[N];
int n,m;
int dis1[N];
int dis2[N];

void dijkstra_second_shortest(int start){//同一个点会被多次访问
	for(int i=1;i<=n;i++){
		dis1[i]=dis2[i]=INF;
	} 
	dis1[start]=0;
	
	priority_queue<Node,vector<Node>,greater<Node> >pq;//存储到达 pos的又一种方式 
							//维护的是已知但未处理的路径信息**********
	pq.push({0,start});
	
	while(!pq.empty()){
		Node cur=pq.top();
		pq.pop();
		int pos=cur.pos;
		int cur_dis=cur.dis;
		
		if(cur_dis>dis2[pos])continue;
		
		for(const Edge& e:edge[pos]){
			int v=e.to;
			int new_dis=cur_dis+e.val;
			
			if(new_dis<dis1[v]){
				dis2[v]=dis1[v];
				dis1[v]=new_dis;
				pq.push({new_dis,v});
			} 
			else if(dis1[v]<new_dis&&new_dis<dis2[v]){
				dis2[v]=new_dis;
				pq.push({new_dis,v});
			}
		}
		
		
	}
}

signed main(){
	IOS
	cin>>n>>m;
	for(int i=1;i<=m;i++){
		int a,b,d;
		cin>>a>>b>>d;
		edge[a].push_back({b,d});
		edge[b].push_back({a,d});
	}
	
	dijkstra_second_shortest(1);
	
	cout<<dis2[n]<<endl;
	return 0;
}