链表的操作:1、构建空表H，插入 {"a","b",",","f"} 遍历输出单链表H 输出单链表H的长度 判断单链表H是否为空 输出单链表H的第3个元素 输出元素c的位置 在第5个位置之前插入元素e后，输出单链表 删除H的第了个元素，输出单链表H

链表的操作：

1. 构建空表H

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next
        
H = ListNode()

2. 插入 {"a","b",",","f"}

H = ListNode()
curr = H
for val in ["a", "b", ",", "f"]:
    curr.next = ListNode(val)
    curr = curr.next

3. 遍历输出单链表H

curr = H.next
while curr:
    print(curr.val, end=" ")
    curr = curr.next

4. 输出单链表H的长度

curr = H.next
count = 0
while curr:
    count += 1
    curr = curr.next
print(count)

5. 判断单链表H是否为空

if H.next:
    print("H is not empty.")
else:
    print("H is empty.")

6. 输出单链表H的第3个元素

curr = H.next
for i in range(2):
    curr = curr.next
print(curr.val)

7. 输出元素c的位置

curr = H.next
pos = 0
while curr:
    pos += 1
    if curr.val == "c":
        print(f"Position of c is {pos}.")
        break
    curr = curr.next
else:
    print("c is not in the list.")

8. 在第5个位置之前插入元素e后，输出单链表

curr = H.next
pos = 0
while curr:
    pos += 1
    if pos == 4:
        new_node = ListNode("e")
        new_node.next = curr.next
        curr.next = new_node
        break
    curr = curr.next

curr = H.next
while curr:
    print(curr.val, end=" ")
    curr = curr.next

9. 删除H的第了个元素，输出单链表H

H.next = H.next.next

curr = H.next
while curr:
    print(curr.val, end=" ")
    curr = curr.next