# Codeforces Round #181 (Div. 2)

Source: Internet
Author: User

B. Coach

Test instructions: Known to have n students, numbering from 1 to N, where N is a multiple of 3, the teacher needs to divide the N students into groups of 3 people. If the class I student is to be in a group with the J student, then the J student must also want to be in a group with the first student (of course, I or J students may also be a group of other K students). Teachers need to meet the students ' requirements when they are grouped. If the group can be implemented according to the requirements, the number of students will be output; otherwise output-1;

Analysis: First of all need to find out the required students to carry out depth-first traversal, if from a student began to traverse the length of more than 3, according to the requirements of students in a group of more than 3 students, then output-1; If exactly 3, then output the number of the 3 students, if less than 3, the need to fill up the students can Then the output number. Because the student's "requirements" are transitive, an adjacency matrix is used to indicate that a student wants to be in a group with another student. Depth-first traversal, creating an array to indicate whether a student has been visited.

`1#include <iostream>2#include <vector>3 using namespacestd;4 BOOLmatrix[ -][ -];5 BOOLvisited[ -];6 intN;7vector<vector<int> >teams;8vector<int>team;9 voidDfsintCurrent )Ten { One Team.push_back (current); AVisited[current] =true; -      for(intj =0; j<n; J + +) -     if(Matrix[current][j] &&!Visited[j]) the Dfs (j); - } - intMain () - { +     intm; -CIN >> N >>m; +memset (Matrix,false,sizeof(matrix)); AMemset (visited,false,sizeof(visited)); at      for(intj =0; j<m; J + +) -     { -         intA, B; -Cin >> a >>b; -Matrix[a-1][b-1] = matrix[b-1][a-1] =true; -     } invector<int>Free ; -      for(intj =0; j<n; J + +) to     { +         BOOLIsfree =true; -          for(intK =0; k<n; k++) the         { *             if(Matrix[j][k]) \$Isfree =false;Panax Notoginseng         } -         if(Isfree) the         { +VISITED[J] =true; A Free.push_back (j); the         } +     } -      for(intj =0; j<n; J + +) \$     if(!Visited[j]) \$     { - team.clear (); - Dfs (j); the         if(Team.size () >3) -         {Wuyicout <<-1; the             return 0; -         } Wu          while(Team.size () <3) -         { About             if(Free.empty ()) \$             { -cout <<-1; -                 return 0; -             } A Team.push_back (Free.back ()); + Free.pop_back (); the         } - Teams.push_back (team); \$     } the     if(Teams.size () <n/3) the      while(!free.empty ()) the     { the team.clear (); -          for(intj =0; j<3; J + +) in         { the Team.push_back (Free.back ()); the Free.pop_back (); About         } the Teams.push_back (team); the     } the      for(intj =0; J<teams.size (); J + +) +     { -          for(intK =0; k<3; k++) thecout << Teams[j][k] +1<<" ";Bayicout <<Endl; the     } the     return 0; -}`
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Codeforces Round #181 (Div. 2)

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