Reverse a stack without using external memory

19.cpp

// but it uses much more memory than normal external memory
void reverse(stk){
  if(!stk.empty()){
    int temp = stk.top();
    stk.pop();
    reverse(stk);
    insert(temp, stk);
  }
}

void insert(item, stk){
  if(stk.empty()){
    stk.push(item);
  }
  else {
    int temp = stk.top();
    stk.pop();
    insert(temp, stk);
  }
}