Problem:
Problem D: Traveling by Stagecoach

Once upon a time, there was a traveler.

He plans to travel using stagecoaches (horse wagons).
His starting point and destination are fixed,
but he cannot determine his route.
Your job in this problem is to write a program
which determines the route for him.

There are several cities in the country,
and a road network connecting them.
If there is a road between two cities, one can travel by a stagecoach
from one of them to the other.
A coach ticket is needed for a coach ride.
The number of horses is specified in each of the tickets.
Of course, with more horses, the coach runs faster.

At the starting point, the traveler has a number of coach tickets.
By considering these tickets and the information on the road network,
you should find the best possible route that takes him to the destination
in the shortest time.
The usage of coach tickets should be taken into account.

The following conditions are assumed.

A coach ride takes the traveler from one city to another
directly connected by a road.
In other words, on each arrival to a city, he must change the coach.

Only one ticket can be used for a coach ride between two cities
directly connected by a road.

Each ticket can be used only once.

The time needed for a coach ride is the distance between two cities
divided by the number of horses.

The time needed for the coach change should be ignored.

Input

The input consists of multiple datasets, each in the following format.
The last dataset is followed by a line containing five zeros
(separated by a space).

n m p a b

t1 t2 ... tn

x1 y1 z1

x2 y2 z2

...

xp yp zp

Every input item in a dataset is a non-negative integer.
If a line contains two or more input items, they are separated by a space.

n is the number of coach tickets.
You can assume that the number of tickets is between 1 and 8.
m is the number of cities in the network.
You can assume that the number of cities is between 2 and 30.
p is the number of roads between cities, which may be zero.

a is the city index of the starting city.
b is the city index of the destination city.
a is not equal to b.
You can assume that all city indices in a dataset (including the above two)
are between 1 and m.

The second line of a dataset gives the details of coach tickets.
ti is the number of horses specified in the i-th
coach ticket (1<=i<=n).
You can assume that the number of horses is between 1 and 10.

The following p lines give the details of roads between cities.
The i-th road connects two cities with city indices
xi and yi,
and has a distance zi (1<=i<=p).
You can assume that the distance is between 1 and 100.

No two roads connect the same pair of cities.
A road never connects a city with itself.
Each road can be traveled in both directions.

Output

For each dataset in the input, one line should be output as specified below.
An output line should not contain extra characters such as spaces.

If the traveler can reach the destination, the time needed
for the best route (a route with the shortest time) should be printed.
The answer should not have an error greater than 0.001.
You may output any number of digits after the decimal point,
provided that the above accuracy condition is satisfied.

If the traveler cannot reach the destination,
the string "Impossible" should be printed.
One cannot reach the destination either when there are no routes
leading to the destination, or when the number of tickets is not sufficient.
Note that the first letter of "Impossible" is in uppercase,
while the other letters are in lowercase.

Sample Input

3 4 3 1 4
3 1 2
1 2 10
2 3 30
3 4 20
2 4 4 2 1
3 1
2 3 3
1 3 3
4 1 2
4 2 5
2 4 3 4 1
5 5
1 2 10
2 3 10
3 4 10
1 2 0 1 2
1
8 5 10 1 5
2 7 1 8 4 5 6 3
1 2 5
2 3 4
3 4 7
4 5 3
1 3 25
2 4 23
3 5 22
1 4 45
2 5 51
1 5 99
0 0 0 0 0

Output for the Sample Input

30.000
3.667
Impossible
Impossible
2.856

Solution in C# (judge verdict: Accepted)

using System.Collections.Generic;
using static System.Math;
using System;

public class PriorityQueue<T> where T : IComparable
{
    private IComparer<T> _comparer = null;
    private int _type = 0;
    private T[] _heap;
    private int _sz = 0;
    private int _count = 0;
    /// <summary>
    /// Priority Queue with custom comparer
    /// </summary>
    public PriorityQueue(int maxSize, IComparer<T> comparer)
    {
        _heap = new T[maxSize];
        _comparer = comparer;
    }

    /// <summary>
    /// Priority queue
    /// </summary>
    /// <param name="maxSize">max size</param>
    /// <param name="type">0: asc, 1:desc</param>
    public PriorityQueue(int maxSize, int type = 0)
    {
        _heap = new T[maxSize];
        _type = type;
    }
    private int Compare(T x, T y)
    {
        if (_comparer != null) return _comparer.Compare(x, y);
        return _type == 0 ? x.CompareTo(y) : y.CompareTo(x);
    }
    public void Push(T x)
    {
        _count++;
        //node number
        var i = _sz++;
        while (i > 0)
        {
            //parent node number
            var p = (i - 1) / 2;

            if (Compare(_heap[p], x) <= 0) break;

            _heap[i] = _heap[p];
            i = p;
        }
        _heap[i] = x;
    }
    public T Pop()
    {
        _count--;
        T ret = _heap[0];
        T x = _heap[--_sz];

        int i = 0;
        while (i * 2 + 1 < _sz)
        {
            //children
            int a = i * 2 + 1;
            int b = i * 2 + 2;

            if (b < _sz && Compare(_heap[b], _heap[a]) < 0) a = b;

            if (Compare(_heap[a], x) >= 0) break;

            _heap[i] = _heap[a];
            i = a;
        }
        _heap[i] = x;
        return ret;
    }
    public int Count()
    {
        return _count;
    }
    public T Peek()
    {
        return _heap[0];
    }
    public bool Contains(T x)
    {
        for (int i = 0; i < _sz; i++) if (x.Equals(_heap[i])) return true;
        return false;
    }
    public void Clear()
    {
        while (this.Count() > 0) this.Pop();
    }
    public IEnumerator<T> GetEnumerator()
    {
        var ret = new List<T>();
        while (this.Count() > 0)
        {
            ret.Add(this.Pop());
        }
        foreach (var r in ret)
        {
            this.Push(r);
            yield return r;
        }
    }
    public T[] ToArray()
    {
        T[] array = new T[_sz];
        int i = 0;
        foreach (var r in this)
        {
            array[i++] = r;
        }
        return array;
    }
}

public class Edge
{
    public int to1 { get; set; }
    public int to2 { get; set; }
    public double d { get; set; }
}
public class Node : IComparable
{
    public int id1 { get; set; }
    public int id2 { get; set; }
    public double d { get; set; }
    public int CompareTo(object obj)
    {
        var x = (Node)obj;
        if (this.d > x.d) return 1;
        else if (this.d == x.d) return 0;
        else return -1;
    }
}

public class hello
{
    public static void Main()
    {
        while (true)
        {
            string[] line = Console.ReadLine().Trim().Split(' ');
            var n = int.Parse(line[0]);
            var m = int.Parse(line[1]);
            var p = int.Parse(line[2]);
            var a = int.Parse(line[3]) - 1;
            var b = int.Parse(line[4]) - 1;
            if (n == 0 && m == 0 && p == 0 && a == -1 && b == -1) break;
            var jmax = 1 << n;
            line = Console.ReadLine().Trim().Split(' ');
            var t = Array.ConvertAll(line, double.Parse);
            var aa = getEdge(n, m, p, t, jmax);
            var mind = new double[m, jmax];
            for (int i = 0; i < m; i++)
                for (int j = 0; j < jmax; j++) mind[i, j] = 9999999999999d;
            goDijk(aa, a, 0, m, jmax, mind);
            print(mind, b, jmax);
        }
    }
    static void print(double[,] mind, int b, int jmax)
    {
        var ans = 9999999999999d;
        for (int i = 0; i < jmax; i++)
            if (mind[b, i] != 9999999999999d) ans = Min(ans, mind[b, i]);
        if (ans == 9999999999999d) Console.WriteLine("Impossible");
        else Console.WriteLine(ans);
    }
    static List<Edge>[,] getEdge(int n, int m, int p, double[] t, int jmax)
    {
        var aa = new List<Edge>[m, jmax];
        for (int i = 0; i < m; i++)
            for (int j = 0; j < jmax; j++) aa[i, j] = new List<Edge>();
        for (int i = 0; i < p; i++)
        {
            string[] line = Console.ReadLine().Trim().Split(' ');
            var x = int.Parse(line[0]) - 1;
            var y = int.Parse(line[1]) - 1;
            var z = double.Parse(line[2]);
            for (int j = 0; j < jmax; j++)
                for (int k = 0; k < n; k++)
                    if (((j >> k) & 1) == 0)
                    {
                        var ww2 = (j | (1 << k));
                        var d = z / t[n - k - 1];
                        aa[x, j].Add(new Edge { d = d, to1 = y, to2 = ww2 });
                        aa[y, j].Add(new Edge { d = d, to1 = x, to2 = ww2 });
                    }
        }
        return aa;
    }

    static void goDijk(List<Edge>[,] aa, int r1, int r2, int m, int jmax, double[,] mind)
    {
        var pq = new PriorityQueue<Node>(jmax + 100000);
        pq.Push(new Node { id1 = r1, id2 = r2, d = 0d });
        while (pq.Count() > 0)
        {
            var targ = pq.Pop();
            var nowd = targ.d;
            var nowid1 = targ.id1;
            var nowid2 = targ.id2;
            if (mind[nowid1, nowid2] < nowd) continue;
            mind[nowid1, nowid2] = nowd;
            foreach (var x in aa[nowid1, nowid2])
                if (nowd + x.d < mind[x.to1, x.to2])
                {
                    mind[x.to1, x.to2] = nowd + x.d;
                    pq.Push(new Node { id1 = x.to1, id2 = x.to2, d = nowd + x.d });
                }
        }
    }
}