Задача со стеком

#include <iostream>
#include <stdexcept>
#include <cstdlib>
#include <ctime>
 
template <class ValueType>
class SimpleStack {
 public:
  explicit SimpleStack(size_t size)
    : size_(size), data_(new ValueType[size]), last_(data_) {}
  ~SimpleStack() {
    delete [] data_;
  }
  void push(const ValueType &value) {
    if (last_ > (data_ + size_))
      throw std::runtime_error("Stack overflow.");
    *last_++ = value;
  }
  ValueType pop() {
    if (last_ <= data_)
      throw std::runtime_error("Stack underflow.");
    return *(--last_);
  }
  const ValueType &head() const {
    if (last_ <= data_)
      throw std::out_of_range("Stack is empty.");
    return *(last_ - 1);
  }
  const size_t size() const {
    return last_ - data_;
  }
  friend std::ostream &operator<<(std::ostream &stream, SimpleStack<ValueType> &stack) {
    if (stack.size() == 0)
      return stream;
    ValueType head = stack.pop();
    stream << stack;
    stream << head << " ";
    stack.push(head);
    return stream;
  }
 private:
  size_t size_;
  ValueType *data_;
  ValueType *last_;
};
 
template <class ValueType>
bool doesStackHaveValue(SimpleStack<ValueType> &stack, const ValueType &value) {
  if (stack.size() == 0)
    return false;
  bool result;
  ValueType head = stack.pop();
  if (head == value)
    result = true;
  else
    result = doesStackHaveValue(stack, value);
  stack.push(head);
  return result;
}
 
template <class ValueType>
void fillStack(SimpleStack<ValueType> &stack,
  SimpleStack<ValueType> &stack1, SimpleStack<ValueType> &stack2) {
  if (stack1.size() == 0)
    return;
  int head = stack1.pop();
  if (!doesStackHaveValue(stack, head) &&
    !doesStackHaveValue(stack2, head))
    stack.push(head);
  fillStack(stack, stack1, stack2);
  stack1.push(head);
}
 
int main(int argc, char *argv[]) {
  srand(time(NULL));
  SimpleStack<int> stack(15), stack1(15), stack2(15);
 
  for (int i = 1; i < 10; ++i) {
    stack1.push(rand() % 10);
    stack2.push(rand() % 10);
  }
 
  std::cout << "Stack 1: " << stack1 << std::endl;
  std::cout << "Stack 2: " << stack2 << std::endl;
  
  fillStack(stack, stack1, stack2);
  
  std::cout << "Stack: " << stack << std::endl;
  
  return 0;
}