View Compilation Error

0_0_35943024_9461.cpp: In function 'int main()':
0_0_35943024_9461.cpp:10:2: error: 'unordered_set' was not declared in this scope
  unordered_set<int> st;
  ^
0_0_35943024_9461.cpp:10:16: error: expected primary-expression before 'int'
  unordered_set<int> st;
                ^
0_0_35943024_9461.cpp:16:18: error: 'st' was not declared in this scope
    for (auto itr=st.begin(); itr!=st.end(); itr++){
                  ^
0_0_35943024_9461.cpp:23:4: error: 'st' was not declared in this scope
    st.clear();
    ^
0_0_35943024_9461.cpp:26:4: error: 'st' was not declared in this scope
    st.insert(n);
    ^