题目连接

http://acm.hdu.edu.cn/showproblem.php?pid=5154

Harry and Magical Computer

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. $1 \leq n \leq 100,1 \leq m \leq 10000$
The next following m lines, each line contains two numbers a b, indicates a dependencies $(a, b). 1 \leq a, b \leq n$

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

先建立一张有向图，再遍历，若有顶点未访问到输出"NO",否则输出"YES"。。。

 #include<algorithm>

 #include<iostream>

 #include<cstdlib>

 #include<cstring>

 #include<cstdio>

 #include<vector>

 #include<queue>

 #include<map>

 using std::cin;

 using std::cout;

 using std::endl;

 using std::find;

 using std::sort;

 using std::pair;

 using std::vector;

 using std::queue;

 using std::multimap;

 #define pb(e) push_back(e)

 #define sz(c) (int)(c).size()

 #define mp(a, b) make_pair(a, b)

 #define all(c) (c).begin(), (c).end()

 #define iter(c) decltype((c).begin())

 #define cls(arr,val) memset(arr,val,sizeof(arr))

 #define cpresent(c, e) (find(all(c), (e)) != (c).end())

 #define rep(i, n) for (int i = 0; i < (int)(n); i++)

 #define tr(c, i) for (iter(c) i = (c).begin(); i != (c).end(); ++i)

 const int N = ;

 int n, m, G[N][N];

 bool vis[N], flag[N];

 typedef unsigned long long ull;

 void bfs() {

     bool f = false;

     queue<int> que;

     rep(i, n) {

         if (!flag[i]) {

             vis[i] = true;

             que.push(i);

         }

     }

     while (!que.empty()) {

         int p = que.front(); que.pop();

         rep(i, n) {

             if (G[p][i]) {

                 if (vis[i]) continue;

                 que.push(i);

                 vis[i] = true;

             }

         }

     }

     rep(i, n) {

         if (!vis[i]) { f = true; break; }

     }

     puts(f ? "NO" : "YES");

 }

 int main() {

 #ifdef LOCAL

     freopen("in.txt", "r", stdin);

     freopen("out.txt", "w+", stdout);

 #endif

     int a, b;

     while (~scanf("%d %d", &n, &m)) {

         cls(vis, ), cls(flag, ), cls(G, );

         rep(i, m) {

             scanf("%d %d", &a, &b);

             G[b - ][a - ] = ;

             flag[a - ] = ;

         }

         bfs();

     }

     return ;

 }

巴特西

hdu 5154 Harry and Magical Computer