View Compilation Error

0_0_25457152_26828.cpp: In function 'int main()':
0_0_25457152_26828.cpp:11:3: error: 'stack' was not declared in this scope
   stack<int> st;
   ^
0_0_25457152_26828.cpp:11:9: error: expected primary-expression before 'int'
   stack<int> st;
         ^
0_0_25457152_26828.cpp:17:11: error: 'st' was not declared in this scope
    while(!st.empty() && st.top()>=x){
           ^
0_0_25457152_26828.cpp:21:4: error: 'st' was not declared in this scope
    st.push(x);
    ^