hdu 5154 Harry and Magical Computer

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 ;
 }