View Compilation Error

0_0_29961608_18991.cpp: In function 'int main()':
0_0_29961608_18991.cpp:51:9: error: 'set' was not declared in this scope
         set<int>s;
         ^
0_0_29961608_18991.cpp:51:13: error: expected primary-expression before 'int'
         set<int>s;
             ^
0_0_29961608_18991.cpp:55:13: error: 's' was not declared in this scope
             s.insert(a[i]);
             ^