View Compilation Error

0_0_26404785_14015.cpp:5:1: error: 'set' does not name a type
 set<int>s;
 ^
0_0_26404785_14015.cpp: In function 'int main()':
0_0_26404785_14015.cpp:11:57: error: 's' was not declared in this scope
     for (int i=1;i<=n;++i) {scanf("%d",&a[i]);if (a[i]) s.insert(a[i]);}
                                                         ^
0_0_26404785_14015.cpp:12:19: error: 's' was not declared in this scope
     printf("%d\n",s.size());
                   ^