C :

#include<stdio.h>
#include<stdlib.h>
#include<malloc.h>

#define ERROR 0
#define OK 1

typedef int Status;
typedef int ElemType;
typedef struct Node
{
	ElemType data;
	struct Node *next;
}Node;
typedef struct Stack
{
	Node *top;
}Stack;

Status Init(Stack *S)
{
	if(!S)
		return ERROR;
	S->top=NULL;
	return OK;
}
Status Empty(Stack S)
{
	return !S.top;
}
Status Push(Stack *S,ElemType e)
{
	Node *newnode=(Node *)malloc(sizeof(Node));
	if(!newnode)
		return ERROR;
	newnode->data=e;
	newnode->next=S->top;
	S->top=newnode;
	return OK;
}
Status Pop(Stack *S,ElemType *e)
{
	Node *p=S->top;
	*e=p->data;
	S->top=p->next;
	free(p);
	return OK;
}
Status Destroy(Stack S,ElemType e)
{
	if(!Empty(S))
		Pop(&S,&e);
	 return OK;
}


Status GetTop(Stack S,ElemType *e)
{
	if(Empty(S))
		return ERROR;
	*e=S.top->data;
	return OK;
}
int main()
{
	int n,i,e;
	char a;
	while((scanf("%d",&n)!=EOF))
	{

		if(n==0)
			return 0;
		Stack S;
		Init(&S);
		for(i=0;i<n;i++)
		{
			scanf("\n%c",&a);
			switch(a)
			{
				case'P':
					{
						scanf("%d",&e);
						Push(&S,e);
						if(i==n-1)
							printf("\n");
						continue;
					}
				case'O':
					{
						if(!Empty(S))
							Pop(&S,&e);
						if(i==n-1)
							printf("\n");
						continue;
					}
				case'A':
					{
						if(GetTop(S,&e))
							printf("%d\n",e);
						else
							printf("E\n");
						if(i==n-1)
							printf("\n");
						continue;
					}	
			}					
		}
		Destroy(S,e);
	}
	return 0;
}

C++ :

#include <stdio.h>
int n;
int run()
{
	int i,a[12222],l=0;
	char c;
	while(n>0)
	{
		n--;
		c=' ';
		while((c!='P')&&(c!='O')&&(c!='A'))
			scanf("%c",&c);
		if(c=='P')
		{
			scanf("%d",&i);
			l++;
			a[l]=i;
		}
		if(c=='O')
			if(l>0)
				l--;
		if(c=='A')
		{
			if(l>0)
				printf("%d\n",a[l]);
			else
				printf("E\n");
		}
	}
	printf("\n");
}
int main()
{
	scanf("%d",&n);
	while(n!=0)
	{
		run();
		n=0;
		scanf("%d",&n);
	}
	return 0;
}

Pascal :

program acm1909;
var
 n,i,t,top:integer; ch:string;
 a:array[1..10000]of longint;
begin
 readln(n);
 while n<>0 do
  begin
   top:=0;
   for i:=1 to n do
    begin
     readln(ch);
     if ch[1]='P' then
      begin
       ch:=copy(ch,2,length(ch));
       val(ch,t);
       inc(top); a[top]:=t;
      end;
     if (ch[1]='O')and(top>0) then dec(top);
     if ch[1]='A' then
      begin
       if top=0 then writeln('E')
        else writeln(a[top]);
      end;
    end;
   writeln;
   readln(n);
  end;
end.

Java :

import java.util.*;
public class Main 
{
	public static void main(String[] args) 
	{
		int a,b,n;
		String k;
		Stack<Integer> s = new Stack<Integer>();
		Scanner cin=new Scanner(System.in);
		while(cin.hasNext()) 
		{
			n=cin.nextInt();
			if(n==0)
				break;
			for(int i=0;i<n;i++)
			{
				k=cin.next();
				if(k.charAt(0)=='P')
				{
					s.push(cin.nextInt());
				}
				else if(k.charAt(0)=='O')
				{
					if(s.empty()!=true)
						s.pop();
				}
				else if(k.charAt(0)=='A')
				{
					if(s.empty()!=true)
						System.out.println(s.peek());
					else
						System.out.println("E");
				}
			}
			while(s.empty()!=true)
				s.pop();
			System.out.print("\n");
		}
		cin.close();
	}
}