Method fix

code/data-structures/monotonic_queue.cpp:13–15

  min_stack inp, outp;
  void push(int x) { inp.push(x); }
  void fix() {
    if (outp.empty()) while (!inp.empty())
      outp.push(inp.top()), inp.pop(); }
  int top() { fix(); return outp.top(); }
  int mn() {
    if (inp.empty()) return outp.mn();