Stack and Queue

#include<stdio.h>
#include<stdlib.h>
#define N 100
struct stack {
  int top;
  int rear;
  int element[N];
};
struct queue {
  int top;
  int rear;
  int element[N];
};
void push(int x, struct stack *S) {
  if (S->top == 0) {
    printf("Error\n");
    exit(1);
  }
  S->element[--S->top] = x;
}
void pop(struct stack *S) {
  if (S->top >= N) {
    printf("Error\n");
    exit(1);
  }
  printf("%d ", S->element[S->top++]);
}
void enqueue(int x, struct queue *Q) {
  if (Q->top == Q->rear) {
    Q->rear = 0;
    Q->top = 0;
    Q->element[Q->rear++] = x;
    return;
  }
  if (Q->rear == N) {
    printf("Error\n");
    exit(1);
  }
  Q->element[Q->rear++] = x;
}
void dequeue(struct queue *Q) {
  if (Q->rear <= Q->top) {
    printf("Error\n");
    exit(1);
  }
  printf("%d ", Q->element[Q->top++]);
}
int main(void) {
  int x;
  struct stack S;
  struct queue Q;
  S.top = N;
  Q.top = Q.rear = 0;
  while (1) {
    printf("整数を入力してください: ");
    scanf("%d", &x);
    if (x < 0) break;
    push(x, &S);
    enqueue(x, &Q);
  }
  printf("スタック構造の場合\n");
  while (S.top < N)
    pop(&S);
  printf("\nキュー構造の場合\n");
  while (Q.top < Q.rear)
    dequeue(&Q);
  printf("\n");
  return 0;
}