Fila dinâmica em C

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/* a simple dynamic queue in C 
       
  functions: 
  - insert and remove element (no)
  - show the queue
  - free queue
  
  writer: Alexandre (azk4n)    
*/

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

typedef struct NO{
    int data;
    struct NO *nxt;
 }no;
 
int size;
no *tmp;

int empty(no *queue){
   if(queue->nxt == NULL) return 1;
   else return 0;
}


void showQueue(no *queue){
   if(empty(queue)){
      printf("\nempty queue\n\n");
   }else{
      tmp = queue->nxt;
      printf("\n");
      while(tmp != NULL){
         printf("%d ", tmp->data);
         tmp = tmp->nxt;
      }printf("\nsize of queue: %d\n\n", size);
   }
}

void enqueue(no *queue){
   no *new = (no*) malloc(sizeof(no));
   new->nxt = NULL;
   printf("\ntake the new node: \n"); 
   scanf("%d", &new->data);
   printf("\n");
   if(empty(queue)){
      queue->nxt = new;
      size++;
   }else{
      tmp = queue->nxt;
      while(tmp->nxt != NULL){
         tmp = tmp->nxt; 
      }
      tmp->nxt = new;
      size++;
   }
}

no *dequeue(no *queue){
   if(empty(queue)){
      printf("\nempty queue\n\n");
      return NULL;
   }else{
      tmp = queue->nxt;
      queue->nxt = tmp->nxt; 
      size--;
      return tmp;
   }
}

void freeQueue(no *queue){
   if(empty(queue)){
      printf("\nempty queue\n\n");
   }else{
      no *nxtNode,
         *actualNode;
      actualNode = queue->nxt;
      while(actualNode != NULL){
         nxtNode = actualNode->nxt;
         free(actualNode);
         actualNode = nxtNode;
      }
      printf("\nqueue clear\n\n");
   }
}
   
int main(){
   no *queue = (no*) malloc(sizeof(no));
   if(!queue){
      printf("no memory");
      exit(1);
   }else{
      queue->nxt = NULL;
      size = 0;
      int opt;
   
      do{   
         printf("0 -> quit\n");
         printf("1 -> show queue\n");
         printf("2 -> add node in queue\n");
         printf("3 -> remove node in queue\n");
         printf("4 -> free queue\n");
         printf("opt: "); scanf("%d", &opt);
      
         switch(opt){
            case 1:
               showQueue(queue);
               break;
            case 2:
               enqueue(queue);
               break;
            case 3:
               tmp = dequeue(queue);
               if(tmp != NULL)
                  printf("\nelement removed: %d\n\n", tmp->data);
                  break;
            case 4:
               freeQueue(queue);
               queue->nxt = NULL;
               size = 0;
               break;
            default:
               if(opt != 0) printf("\ninsert valid option!\n\n"); 
          }   
      }while(opt != 0);
   }
   return 1;
}