stack.c

#include<stdio.h>
#include<stdlib.h>
typedef int it; //various type


struct stack_s{
  it x;
  struct stack_s * before;
  struct stack_s * next;
};

struct my_stack{
  int len;
  struct stack_s * last;
};

it top(struct my_stack * st){
  if(st -> len == 0)return -1;
  return  st -> last -> x;
}

void push(struct my_stack * st, it val){
  if(st->len != 0){
    st -> last -> next           = (struct  stack_s *)malloc(sizeof(struct stack_s));
    st -> last -> next -> before = st -> last;
    st -> last                   = st -> last -> next;
    st -> last -> x              = val;
    st -> last -> next           = 0;
    st -> len                   += 1;
  }
  else{
    st -> last                   = (struct stack_s *) malloc(sizeof(struct stack_s));
    st -> last -> x              = val;
    st -> last -> before         = 0;
    st -> last -> next           = 0;
    st -> len                   += 1;
  }
}

int pop(struct my_stack * st){
  if(st -> len == 0)return 0;
  if(st -> len == 1){
    st -> last = 0;
    st -> len -= 1;
  }
  else{
    st -> last = st -> last -> before;
    st -> len -= 1;  
    free(st -> last -> next);
  }
  return 1;
}

int len(struct my_stack * st){
  return st -> len;
}

int main(){
  int i,n;
  struct my_stack test;
  test . len = 0;
  while(1){
    scanf("%d",&n);
    if(n == -1)return 0;
    if(n == 0)printf("%d, %d\n",pop(&test) ,top(&test));
    if(n >  0)push(&test,n);
  }
}

stack_without_len.c

#include<stdio.h>
#include<stdlib.h>
typedef int it;


struct stack_s{
  it x;
  struct stack_s * before;
};

struct my_stack{
  struct stack_s * last;
};

it top(struct my_stack * st){
  if(st -> last  == 0)return -1;
  return  st -> last -> x;
}

void push(struct my_stack * st, it val){
  if(st -> last != 0){
    struct stack_s * before      = st -> last;
    st -> last                   = (struct stack_s *)malloc(sizeof(struct stack_s));
    st -> last -> before         = before;
    st -> last -> x              = val;
  }
  else{
    st -> last                   = (struct stack_s *) malloc(sizeof(struct stack_s));
    st -> last -> x              = val;
    st -> last -> before         = 0;
  }
}

int pop(struct my_stack * st){
  if(st -> last == 0)return 0;
  if(st -> last -> before  == 0){
    st -> last = 0;
  }
  else{
    struct stack_s * next = st -> last;
    st -> last = st -> last -> before;
    free(next);
  }
  return 1;
}


int main(){
  int i,n;
  struct my_stack test;
  test . last = 0;
  while(1){
    scanf("%d",&n);
    if(n == -1)return 0;
    if(n == 0)printf("%d, %d\n",pop(&test) ,top(&test));
    if(n >  0)push(&test,n);
  }
}