POJ1129

Channel Allocation

Time Limit: 1000MS Memory Limit: 10000KTotal Submissions: 12311 Accepted: 6296

Description

When a radio station is broadcasting over a very large area, repeaters are used to retransmit the signal so that every receiver has a strong signal. However, the channels used by each repeater must be carefully chosen so that nearby repeaters do not interfere with one another. This condition is satisfied if adjacent repeaters use different channels. 

Since the radio frequency spectrum is a precious resource, the number of channels required by a given network of repeaters should be minimised. You have to write a program that reads in a description of a repeater network and determines the minimum number of channels required.

Input

The input consists of a number of maps of repeater networks. Each map begins with a line containing the number of repeaters. This is between 1 and 26, and the repeaters are referred to by consecutive upper-case letters of the alphabet starting with A. For example, ten repeaters would have the names A,B,C,...,I and J. A network with zero repeaters indicates the end of input. 

Following the number of repeaters is a list of adjacency relationships. Each line has the form: 

A:BCDH 

which indicates that the repeaters B, C, D and H are adjacent to the repeater A. The first line describes those adjacent to repeater A, the second those adjacent to B, and so on for all of the repeaters. If a repeater is not adjacent to any other, its line has the form 

A: 

The repeaters are listed in alphabetical order. 

Note that the adjacency is a symmetric relationship; if A is adjacent to B, then B is necessarily adjacent to A. Also, since the repeaters lie in a plane, the graph formed by connecting adjacent repeaters does not have any line segments that cross. 

Output

For each map (except the final one with no repeaters), print a line containing the minumum number of channels needed so that no adjacent channels interfere. The sample output shows the format of this line. Take care that channels is in the singular form when only one channel is required.

Sample Input

2A:B:4A:BCB:ACDC:ABDD:BC4A:BCDB:ACDC:ABDD:ABC0

Sample Output

1 channel needed.3 channels needed.4 channels needed. 

注：其实更像递归， 每个站枚举信号，从1开始，如果它相邻的排在他前面的站的信号和当前枚举到的信号相同，则跳到下一个信号，依次类推，直到最后一个站有信号，每次记录当前已经使用的数值最大的信号，则最后这个最大的信号就是结果！

#include<iostream>#include<string>using namespace std;int n,minc,maxc;string s[26];int num[26];int dfs(int m,int x){    if(m==n){      if(x<minc) minc=x;      return 1;    }else {for(int i=1;i<=m+1;i++){        int j;    for( j=2;j<m+2;j++){       if(s[m][j]<s[m][0]&&num[s[m][j]-'A']==i) {j=0;break;}    }   if(j==0) continue;   num[m]=i;   if(i>x) x=i;    if(dfs(m+1,x)) break;} }}int main(void){while(cin>>n&&n){        for(int i=0;i<n;i++) cin>>s[i];minc=30; dfs(0,0); if(minc==1)  cout<<minc<<" channel needed."<<endl;else cout<<minc<<" channels needed."<<endl;}}