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"。。。

 1 #include<algorithm>

 2 #include<iostream>

 3 #include<cstdlib>

 4 #include<cstring>

 5 #include<cstdio>

 6 #include<vector>

 7 #include<queue>

 8 #include<map>

 9 using std::cin;

10 using std::cout;

11 using std::endl;

12 using std::find;

13 using std::sort;

14 using std::pair;

15 using std::vector;

16 using std::queue;

17 using std::multimap;

18 #define pb(e) push_back(e)

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

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

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

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

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

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

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

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

27 const int N = 110;

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

29 bool vis[N], flag[N];

30 typedef unsigned long long ull;

31 void bfs() {

32     bool f = false;

33     queue<int> que;

34     rep(i, n) {

35         if (!flag[i]) {

36             vis[i] = true;

37             que.push(i);

38         }

39     }

40     while (!que.empty()) {

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

42         rep(i, n) {

43             if (G[p][i]) {

44                 if (vis[i]) continue;

45                 que.push(i);

46                 vis[i] = true;

47             }

48         }

49     }

50     rep(i, n) {

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

52     }

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

54 }

55 int main() {

56 #ifdef LOCAL

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

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

59 #endif

60     int a, b;

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

62         cls(vis, 0), cls(flag, 0), cls(G, 0);

63         rep(i, m) {

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

65             G[b - 1][a - 1] = 1;

66             flag[a - 1] = 1;

67         }

68         bfs();

69     }

70     return 0;

71 }

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