exp 3,5,6,7: init

Signed-off-by: Amneesh Singh <natto@weirdnatto.in>

3.c

@@ -0,0 +1,76 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+typedef struct Employee employee;
+struct Employee {
+  int eid;
+  struct Employee *next;
+  struct Employee *prev;
+};
+
+typedef struct Employees employees;
+struct Employees {
+  uint len;
+  struct Employee *head;
+  struct Employee *tail;
+};
+
+employees *init() {
+  employees *list = malloc(sizeof(employees));
+  list->len = 0;
+  list->head = NULL;
+  list->tail = NULL;
+
+  return list;
+}
+
+void insert(employees *list, uint eid) {
+  employee *new = malloc(sizeof(employee));
+  new->eid = eid;
+  new->prev = NULL;
+  new->next = list->head;
+
+  if (list->head)
+    list->head->prev = new;
+  else
+    list->tail = new;
+
+  list->head = new;
+  list->len++;
+}
+
+void delete (employees *list) {
+  employee *prev = list->tail->prev;
+
+  free(list->tail);
+
+  if (prev) {
+    prev->next = NULL;
+    if (prev->prev == NULL)
+      list->head = prev;
+  }
+
+  list->tail = prev;
+  list->len--;
+}
+
+int main() {
+  employees *list = init();
+  insert(list, 4);
+  insert(list, 20);
+  insert(list, 300);
+  insert(list, 44);
+  delete (list);
+
+  employee *cur = list->head;
+  while (cur != NULL) {
+    printf("%d ", cur->eid);
+    cur = cur->next;
+  }
+
+  // free
+  while (list->tail != NULL)
+    delete (list);
+
+  free(list);
+}