2012多校联盟第二场1009 还是BFS

开始用DFS，交了超时。后来想是不是没有对访问过的节点进行标记，于是开个标记数组。交了，WR。发现没有输出“IMPOSSIBLE”的情况，改完交，又WR。后来换BFS，终于A了，又完成这周的“任务”了。记得上周也是过1009的BFS题，汗！！！

下面附题目：

Power transmission

Time Limit : 10000/5000ms (Java/Other)   Memory Limit : 131072/131072K (Java/Other)

Total Submission(s) : 13   Accepted Submission(s) : 2

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Problem Description

The project West-East power transmission is famous around the world. It transmits the electricity from western areas to east China. There are many nodes in the power system. Each node is connected with several other nodes in the system by cable. Power can be only transmitted between two connected nodes. For each node, it can’t send power to two or more other nodes at the same time. As we have all known, power will be loss during the transmission. Bob is the chief engineer of the project. He wants to build a transmission line which send power from one node to another node and minimize the power loss at the same time. Now he asks you to help him solve the problem.

Input

There are several test cases. For each test case, the first line contains an integer N (0 < N ≤ 50000) which represents the number of nodes in the power system. Then there will be N groups of data following. For the i-th(0 < i ≤ N) group, the first line is an integer ki (ki ≤ 50), which means the node i is connected with ki nodes. The rest of the i-th group data are divided into ki lines. Each line contains an integer ai (0 < ai ≤ N, ai ≠ i) and an integer bi (0 ≤ bi ≤ 100), which represents power can be transmitted from node i to ai and will loss bi% while transmitting. The last line of input data contains three integers separated by single spaces. The first one is s, the second is t (0 < s, t ≤ N), and the third is the total power M (0 < M ≤ 10^6) at node s.

Output

For each test case, output the minimum of loss power while transmitting from node s to node t. The result should be printed with two digits to the right of the decimal point. If power cannot be transmitted from node s to node t, output “IMPOSSIBLE!” in a line.

Sample Input

4
2
2 50
3 70
2
1 30
4 20
2
1 10
4 40
0
1 4 100

Sample Output

60.00

Hint

In the sample, the best transmission line is 1 -> 2 -> 4, loss power is 100 * 50% + 100 * (100%-50%)*20% = 60.00

Author

TJU

Source

2012 Multi-University Training Contest 2