HDU 5154 Harry and Magical Computer（找环）

Harry and Magical Computer

Time Limit: 2000/1000 MS (Java/Others)    Memory Limit: 32768/32768 K (Java/Others)
Total Submission(s): 1743    Accepted Submission(s): 684

Problem Description

In reward of being yearly outstanding magic student, Harry gets a magical computer. When the computer begins to deal with a process, it will work until the ending of the processes. One day the computer got n processes to deal with. We number the processes from 1 to n. However there are some dependencies between some processes. When there exists a dependencies (a, b), it means process b must be finished before process a. By knowing all the m dependencies, Harry wants to know if the computer can finish all the n processes.

 

Input

There are several test cases, you should process to the end of file.
For each test case, there are two numbers n m on the first line, indicates the number processes and the number of dependencies.  1≤n≤100,1≤m≤10000
The next following m lines, each line contains two numbers a b, indicates a dependencies (a, b).  1≤a,b≤n

 

Output

Output one line for each test case. 
If the computer can finish all the process print "YES" (Without quotes).
Else print "NO" (Without quotes).

 

Sample Input

3 2 3 1 2 1 3 3 3 2 2 1 1 3

 

Sample Output

YES NO

 

Source

BestCoder Round #25

 

问题描述

作为年度优秀魔法学员的奖赏，哈利得到了一台具有魔力的计算机。这台计算机一旦开始处理某个任务，就会一直处理到这个任务结束为止(所以你可以认为它是单线程的)。有一天，这台计算机得到了n个任务要处理，分别标号1到n。这n个任务之间又有一些依赖关系，假如存在依赖关系(a, b)，那么要处理a任务，必须先将b任务完成。现在哈利得到了所有的这些依赖关系，一共m个。他想知道，这台计算机能否完成所有的任务。

输入描述

多组输入数据
每组数据第一行由n m组成，分别代表任务数和依赖关系数。$1\le n\le 100,1\le m\le 10000$
接下来m行，每行两个数a b，表示一组依赖关系。$1\le a,b\le n$

输出描述

每组输出一行，"YES"(没有引号)表示能完成所有任务。
"NO"(没有引号)表示不能。

输入样例

3 2
3 1
2 1
3 3
3 2
2 1
1 3

输出样例

YES
NO

解题思路：如果存在任务不能完成，那么说明该任务既出现在某任务的前面又出现在某任务的后面，换句话说就是某些任务与该任务一起组成了一个环。用弗洛伊德算法，判断最后的邻接矩阵的对角线是否有1存在，如果有，说明存在环，否则没有。

代码如下：

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include