【CodeForces 129 B】Students and Shoelaces（拓扑排序）

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Anna and Maria are in charge of the math club for junior students. When the club gathers together, the students behave badly. They‘ve brought lots of shoe laces to the club and got tied with each other. Specifically, each string ties together two students. Besides, if two students are tied, then the lace connects the first student with the second one as well as the second student with the first one.

To restore order, Anna and Maria do the following. First, for each student Anna finds out what other students he is tied to. If a student is tied to exactly one other student, Anna reprimands him. Then Maria gathers in a single group all the students who have been just reprimanded. She kicks them out from the club. This group of students immediately leaves the club. These students takes with them the laces that used to tie them. Then again for every student Anna finds out how many other students he is tied to and so on. And they do so until Anna can reprimand at least one student.

Determine how many groups of students will be kicked out of the club.

Input

The first line contains two integers n and m — the initial number of students and laces (). The students are numbered from 1 to n, and the laces are numbered from 1 to m. Next m lines each contain two integers a and b — the numbers of students tied by the i-th lace (1 ≤ a, b ≤ n, a ≠ b). It is guaranteed that no two students are tied with more than one lace. No lace ties a student to himself.

Output

Print the single number — the number of groups of students that will be kicked out from the club.

Examples

3 3
1 2
2 3
3 1

0

6 3
1 2
2 3
3 4

2

6 5
1 4
2 4
3 4
5 4
6 4

1

Note

In the first sample Anna and Maria won‘t kick out any group of students — in the initial position every student is tied to two other students and Anna won‘t be able to reprimand anyone.

In the second sample four students are tied in a chain and two more are running by themselves. First Anna and Maria kick out the two students from both ends of the chain (1 and 4), then — two other students from the chain (2 and 3). At that the students who are running by themselves will stay in the club.

In the third sample Anna and Maria will momentarily kick out all students except for the fourth one and the process stops at that point. The correct answer is one.

题意：

Anna和Maria要给小学生发鞋带，如果某位小学生的鞋带只跟一个人绑定，那就要踢出去，单独没和人绑定的不予理会，问最少需要几轮可以使所有人都是孤立的。

思路：

每次先处理鞋带与外界绑定数为1的，由于是无向图，无所谓出度入度，进行拓扑排序的同时，计算所进行的总轮数。

#include<bits/stdc++.h>
using namespace std;
int deg[105],n,m;
vector<int>to[105];
int topsort()
{
    queue<int>qu;
    int i,go=1,sum=0;
    while(go)
    {
        go=0;
        for(i=1;i<=n;i++) 
            if(deg[i]==1)
            {
                qu.push(i);
                go=1;
            }
        while(!qu.empty())
        {
            int a=qu.front();
            qu.pop();
            deg[a]--;
            for(i=0;i<to[a].size();i++)
                deg[to[a][i]]--;
        }    
        if(go)sum++;
    }
    return sum;
} 
int main()
{
    ios::sync_with_stdio(false);
    int u,v;
    int i,j,k;
    cin>>n>>m;
    for(i=1;i<=m;i++)
    {
        cin>>u>>v;
        deg[u]++;deg[v]++;
        to[u].push_back(v);
        to[v].push_back(u);
    }
    cout<<topsort()<<endl;
    return 0;
} 

【CodeForces 129 B】Students and Shoelaces（拓扑排序）

标签：stay   get   ecif   nes   sum   can   queue   front   ble   

原文地址：https://www.cnblogs.com/kannyi/p/9784850.html