Egiht（八種方法）

Problem Description

The 15-puzzle has been around for over 100 years; even if you don't know it by that name, you've seen it. It is constructed with 15 sliding tiles, each with a number from 1 to 15 on it, and all packed into a 4 by 4 frame with one tile missing. Let's call the missing tile 'x'; the object of the puzzle is to arrange the tiles so that they are ordered as:

1  2  3  4
 5  6  7  8
 9 10 11 12
13 14 15  x      

where the only legal operation is to exchange 'x' with one of the tiles with which it shares an edge. As an example, the following sequence of moves solves a slightly scrambled puzzle:

1  2  3  4     1  2  3  4     1  2  3  4     1  2  3  4
 5  6  7  8     5  6  7  8     5  6  7  8     5  6  7  8
 9  x 10 12     9 10  x 12     9 10 11 12     9 10 11 12
13 14 11 15    13 14 11 15    13 14  x 15    13 14 15  x
            r->            d->            r->      

The letters in the previous row indicate which neighbor of the 'x' tile is swapped with the 'x' tile at each step; legal values are 'r','l','u' and 'd', for right, left, up, and down, respectively.

Not all puzzles can be solved; in 1870, a man named Sam Loyd was famous for distributing an unsolvable version of the puzzle, and

frustrating many people. In fact, all you have to do to make a regular puzzle into an unsolvable one is to swap two tiles (not counting the missing 'x' tile, of course).

In this problem, you will write a program for solving the less well-known 8-puzzle, composed of tiles on a three by three

arrangement.

Input

You will receive, several descriptions of configuration of the 8 puzzle. One description is just a list of the tiles in their initial positions, with the rows listed from top to bottom, and the tiles listed from left to right within a row, where the tiles are represented by numbers 1 to 8, plus 'x'. For example, this puzzle

1 2 3

x 4 6

7 5 8

is described by this list:

1 2 3 x 4 6 7 5 8

Output

You will print to standard output either the word ``unsolvable'', if the puzzle has no solution, or a string consisting entirely of the letters 'r', 'l', 'u' and 'd' that describes a series of moves that produce a solution. The string should include no spaces and start at the beginning of the line. Do not print a blank line between cases.

SampleInput

2  3  4  1  5  x  7  6  8      

SampleOutput

ullddrurdllurdruldr      

就是給你一個序列，然後要你通過四種移動方式，回到最初的序列，輸出答案，題目有SPJ，無需輸出最優方法。
這道題可謂是搜尋中的精髓了，最開始我就是一個簡單的雙向搜尋，結果TLE了，看了網上的部落格加了剪枝之後過了。
但是後來和隊友聊這題，發現這道題的解題思路不止一種，隊友說有八種      

然後找到一篇部落格，裡面一共講了八種解法，應該是很全面的。
部落格連結：        https://www.cnblogs.com/zufezzt/p/5659276.html               至于我的代碼就沒什麼好講的了，确實用雙向BFS加剪枝可以AC這道題，但這道題的精髓遠遠不在于此，也見識到了自己的學識淺薄。我也道不出個是以然，是以這道題我就不詳細講了。
還是那句話以夢為馬，不負韶華。