View Compilation Error

0_0_21961705_2534.cpp:25:1: error: 'set' does not name a type
 set<int>s[MAXN];
 ^
0_0_21961705_2534.cpp: In function 'int query(int, int)':
0_0_21961705_2534.cpp:27:8: error: 's' was not declared in this scope
     if(s[u].size()>s[v].size())swap(u,v);
        ^
0_0_21961705_2534.cpp:28:2: error: 'set' was not declared in this scope
  set<int>::iterator it;
  ^
0_0_21961705_2534.cpp:28:6: error: expected primary-expression before 'int'
  set<int>::iterator it;
      ^
0_0_21961705_2534.cpp:30:9: error: 'it' was not declared in this scope
     for(it=s[u].begin();it!=s[u].end();it++){
         ^
0_0_21961705_2534.cpp:30:12: error: 's' was not declared in this scope
     for(it=s[u].begin();it!=s[u].end();it++){
            ^
0_0_21961705_2534.cpp: In function 'int main()':
0_0_21961705_2534.cpp:39:41: error: 's' was not declared in this scope
         for(int i=1;i<=n;i++)head[i]=-1,s[i].clear();
                                         ^
0_0_21961705_2534.cpp:44:13: error: 's' was not declared in this scope
             s[u].insert(v);
             ^