栈结构的链表实现方式

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

#define ElementType int
#define ERROR -1

/*
 用链表表示栈
 该栈初始是指向一个空节点
 栈插入删除操作都是在链表头部形成栈结构
 栈头一直指向链表第一个节点
 */

typedef struct SNode *PtrToSNode;
struct SNode{
    ElementType Data;
    PtrToSNode Next;
};
typedef PtrToSNode Stack;

Stack CreateStack(){
    Stack S;
    S = (Stack)malloc(sizeof(struct SNode));
    S->Next = NULL;
    return S;
}

bool IsEmpty(Stack S){
    return (S->Next == NULL);
}

bool Push(Stack S, ElementType X){
    PtrToSNode q;
    q = (PtrToSNode)malloc(sizeof(struct SNode));
    q->Data = X;
    q->Next = S->Next;
    S->Next = q;
    return true;
}

ElementType Pop(Stack S){
    PtrToSNode FirstCell;
    ElementType TopElem;
    
    if(IsEmpty(S)){
        printf("Stack is Empty\n");
        return ERROR;
    }else{
        FirstCell = S->Next;
        TopElem = FirstCell->Data;
        S->Next = FirstCell->Next;
        free(FirstCell);
        return TopElem;
    }
}
int main(){
    ElementType a;
    Stack s = CreateStack();
    Push(s, 0);
    Push(s, 1);
    a = Pop(s);
    printf("a = %d\n",a);
    Push(s, 2);
    a = Pop(s);
    printf("a = %d\n",a);
    a = Pop(s);
    printf("a = %d\n",a);
    a = Pop(s);
    return 0;
}