#include <stdio.h>
#include <string.h>
#define NIL (0)
class ListNode {
public:
	char data;
	ListNode *next;
	ListNode(char d) { data = d; next = NIL; }
};
class Stack {
	ListNode *_top;
	ListNode *_tail;
public: 
	Stack() { _top = NIL; _tail = NIL; }
	void insert(char data) {
		if (_top == NIL) {
			_top = new ListNode(data);
			_tail = _top;
		} else if (_top->next == NIL) {
			_top->next = new ListNode(data);
		} else if (_top->next->next == NIL) {
			ListNode *p = _top->next;
			_top->next = new ListNode(data);
			_top->next = p;
			_top = _top->next;
		} else {
			_top->next = new ListNode(data);
			_tail = _top;
		} 
	}
	char print() {
		if (_tail == NIL) return NIL;
		char c = _tail->data;
		putchar(c);	
		_tail = _tail->next;
		return c;
	}
};
void main()
{
	Stack aStack;
	char *s = strdup("9214757");
	for (int i = 0; i < strlen(s); i++) {
		aStack.insert(s[i]);
	}
	while (aStack.print() != NIL);
	putchar('\n');
}