HDU 3342 图论

 

Legal or Not

Time Limit: 2000/1000 MS (Java/Others)    Memory Limit: 32768/32768 K (Java/Others)
Total Submission(s): 1522    Accepted Submission(s): 641

Problem Description

ACM-DIY is a large QQ group where many excellent acmers get together. It is so harmonious that just like a big family. Every day,many "holy cows" like HH, hh, AC, ZT, lcc, BF, Qinz and so on chat on-line to exchange their ideas. When someone has questions, many warm-hearted cows like Lost will come to help. Then the one being helped will call Lost "master", and Lost will have a nice "prentice". By and by, there are many pairs of "master and prentice". But then problem occurs: there are too many masters and too many prentices, how can we know whether it is legal or not?

We all know a master can have many prentices and a prentice may have a lot of masters too, it's legal. Nevertheless，some cows are not so honest, they hold illegal relationship. Take HH and 3xian for instant, HH is 3xian's master and, at the same time, 3xian is HH's master,which is quite illegal! To avoid this,please help us to judge whether their relationship is legal or not.

Please note that the "master and prentice" relation is transitive. It means that if A is B's master ans B is C's master, then A is C's master.

 

Input

The input consists of several test cases. For each case, the first line contains two integers, N (members to be tested) and M (relationships to be tested)(2 <= N, M <= 100). Then M lines follow, each contains a pair of (x, y) which means x is y's master and y is x's prentice. The input is terminated by N = 0.
TO MAKE IT SIMPLE, we give every one a number (0, 1, 2,..., N-1). We use their numbers instead of their names.

 

Output

For each test case, print in one line the judgement of the messy relationship.
If it is legal, output "YES", otherwise "NO".

 

Sample Input

3 20 11 22 20 11 00 0

 

Sample Output

YESNO

 

Author

QiuQiu@NJFU

 

Source

HDOJ Monthly Contest – 2010.03.06

 

第一眼看到这个题的时候表示木有想到topsort

看到这个题直接就想到可以先求解一个强连通分量

求解完成之后，看下分量个数和原图点的个数是不是相等就可以了1Y

 

我的代码：

#include<stdio.h>#include<string.h>#include<algorithm>#include<stack>#include<vector>#define maxn 105using namespace std;int n,m,Index,cnt;int belong[maxn],dfn[maxn],low[maxn];bool used[maxn],instack[maxn];stack<int>s;vector<int>map[maxn];void init(){int i;for(i=0;i<maxn;i++)map[i].clear();while(!s.empty())s.pop();memset(used,0,sizeof(used));memset(instack,0,sizeof(instack));memset(dfn,-1,sizeof(dfn));memset(low,0,sizeof(low));memset(belong,0,sizeof(belong));Index=0,cnt=0;}int min(int a,int b){if(a>b)return b;elsereturn a;}void tarjan(int u){int i,v;Index++;dfn[u]=Index;low[u]=Index;used[u]=true;instack[u]=true;s.push(u);for(i=0;i<map[u].size();i++){v=map[u][i];if(!used[v]){tarjan(v);low[u]=min(low[u],low[v]);}else if(instack[v]){low[u]=min(low[u],dfn[v]);}}if(dfn[u]==low[u]){cnt++;do{v=s.top();s.pop();belong[v]=cnt;instack[v]=false;}while(u!=v);}}int main(){int i,a,b;while(scanf("%d%d",&n,&m)!=EOF){if(n==0&&m==0)break;init();for(i=1;i<=m;i++){scanf("%d%d",&a,&b);map[a].push_back(b);}for(i=0;i<n;i++)if(dfn[i]==-1)tarjan(i);if(cnt==n)printf("YES\n");elseprintf("NO\n");}return 0;}