[백준 16681] 등산 (Dijkstra) (C/C++) (★★)

#include <stdio.h>
#include <iostream>
#include <vector>
#include <queue>
using namespace std;
 
#define INF 1000000000000
 
vector<pair<int, int>> graph[100010];
long long d_up[100010];
long long d_down[100010];
int height[100010];
int V, E;
int happy, strength;
 
void dijkstra_up(int start)
{
    priority_queue<pair<long long, int>, vector<pair<long long, int>>, greater<pair<long long, int>>> pq;
    pq.push({0, start});
    d_up[start] = 0;
    
    while(!pq.empty())
    {
        int now = pq.top().second;
        long long nowCost = pq.top().first;
        pq.pop();
        
        if(nowCost != d_up[now])
        {
            continue;    
        }
        
        for(int i = 0; i < graph[now].size(); i++)
        {
            int next = graph[now][i].first;
            long long nextCost = graph[now][i].second;
            
            if(height[now] < height[next])
            {
                if(nowCost + nextCost < d_up[next])
                {
                    d_up[next] = nowCost + nextCost;
                    pq.push({d_up[next], next});    
                }    
            }    
        }
    }
}
 
void dijkstra_down(int start)
{
    priority_queue<pair<long long, int>, vector<pair<long long, int>>, greater<pair<long long, int>>> pq;
    pq.push({0, start});
    d_down[start] = 0;
    
    while(!pq.empty())
    {
        int now = pq.top().second;
        long long nowCost = pq.top().first;
        pq.pop();
        
        if(nowCost != d_down[now])
        {
            continue;    
        }
        
        for(int i = 0; i < graph[now].size(); i++)
        {
            int next = graph[now][i].first;
            long long nextCost = graph[now][i].second;
            
            if(height[now] < height[next])
            {
                if(nowCost + nextCost < d_down[next])
                {
                    d_down[next] = nowCost + nextCost;
                    pq.push({d_down[next], next});
                }    
            }    
        }
    }
}
 
int main(void)
{
//    freopen("B16681_input.txt", "r", stdin);
    
    cin >> V >> E >> strength >> happy;
    
    for(int i = 1; i <= V; i++)
    {
        cin >> height[i];
        
        d_up[i] = INF;
        d_down[i] = INF;
    }
 
    for(int i = 1; i <= E; i++)
    {
        int from, to, weight;
        cin >> from >> to >> weight;
        
        graph[from].push_back({to, weight});
        graph[to].push_back({from, weight});
    }
    
    dijkstra_up(1);
    dijkstra_down(V);
    
    long long Max = -INF;
    for(int i = 2; i < V; i++)
    {
        if(Max < happy*height[i] - strength*d_up[i] - strength*d_down[i])
        {
            Max = happy*height[i] - strength*d_up[i] - strength*d_down[i];
        }
    }
    
    if(Max == -INF)
    {
        cout << "Impossible";
    }
    else
    {
        cout << Max;    
    }
    
    return 0;
}