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queue.c
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queue.c
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#include <stdio.h>
#include <stdlib.h>
typedef struct node {
int value;
struct node *prev;
struct node *next;
}node;
node *head, *tail;
void push(int value) {
if (head == NULL) { // if queue is empty, set head node and set head.prev to head to indicate existence of one item
head = malloc(sizeof(node));
head->value = value;
}
else if (tail == NULL) { // if head.prev == head, one item exists
tail = malloc(sizeof(node));
head->next = tail;
tail->value = value;
tail->prev = head;
}
else {
tail->next = malloc(sizeof(node));
tail->next->value = value;
tail->next->prev = tail;
tail = tail->next;
}
}
int pop() {
if (head == NULL) { // return -1 if trying to pop an empty list
return -1;
}
node *temp_addr = head->next;
int result = head->value;
free(head);
if (temp_addr == NULL) {
head = NULL;
tail = NULL;
return result;
}
head = temp_addr;
head->prev = NULL;
return result;
}
int length() {
if (head == NULL) {
return 0;
}
node *current = head;
int i = 1;
while (current->next != NULL) {
i++;
current = current->next;
}
return i;
}
int main() {
push(1);
push(2);
push(3);
printf("%d\n", length());
printf("%d\n", pop());
printf("%d\n", pop());
printf("%d\n", pop());
printf("%d\n", length());
push(3);
printf("%d\n", pop());
printf("%d\n", pop());
}