HDU 3342

 

Legal or Not

Time Limit: 2000/1000 MS (Java/Others)    Memory Limit: 32768/32768 K (Java/Others)
Total Submission(s): 2791    Accepted Submission(s): 1261

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

 

 

题意：a是b的老师，b是c的老师，那么a是c的老师。如果a是b的老师，b是c的老师 ，c是a的老师，或a,b互为老师  这样是不合法的。

思路：拓扑排序。(判断是否有回路)

#include<stdio.h>#include<string.h>#define LEN 110int mp[LEN][LEN];int ru[LEN];int s[LEN];//判定一个点是否判断过入度值  数组初始化为0，若一点i被判断过入度 则s[i]=1;int N, M;int temp;int findZero(){    int i;    int a = -1;    for(i = 0; i < N; i++)        if(s[i] == 0 && ru[i] == 0)        {            a = i;            break;        }    if(a == -1)        return 0;    temp = a;//存储已经找到的入度为0的点的数组下标    return 1;}int TopSort()//拓扑排序{    int i, j;    for(i = 0; i < N; i++)    {        if(findZero())        {            s[temp] = 1;            for(j = 0; j < N; j++)                ru[j] -= mp[temp][j];        }        else            return 0;    }    return 1;}int main(){    int i, j;    int x, y;    while(scanf("%d%d", &N, &M), N)    {        memset(mp, 0, sizeof(mp));        memset(s, 0, sizeof(s));        memset(ru, 0, sizeof(ru));        for(i = 0; i < M; i++)        {            scanf("%d%d", &x, &y);            if(mp[x][y] == 0)            {                mp[x][y] = 1;                ru[y]++;            }        }        if(TopSort() == 1)            printf("YES\n");        else            printf("NO\n");    }    //system("pause");}