//创建一个有头链表 //判空 //求长度 //按位置插入 //显示 //按位置删除 //按数据查找 //按数据修改 //清空 //倒置(逆序) //排序

以下是创建一个有头链表的Python代码，以及链表的基本操作：

# 创建一个有头链表
class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

head = ListNode()

# 判空
def is_empty(head):
    return head.next == None

# 求长度
def get_length(head):
    cur = head.next
    count = 0
    while cur:
        count += 1
        cur = cur.next
    return count

# 按位置插入
def insert(head, pos, val):
    if pos < 0 or pos > get_length(head):
        return False
    cur = head
    for i in range(pos):
        cur = cur.next
    node = ListNode(val)
    node.next = cur.next
    cur.next = node
    return True

# 显示
def show(head):
    cur = head.next
    while cur:
        print(cur.val, end=' ')
        cur = cur.next
    print()

# 按位置删除
def delete_by_pos(head, pos):
    if pos < 0 or pos >= get_length(head):
        return False
    cur = head
    for i in range(pos):
        cur = cur.next
    cur.next = cur.next.next
    return True

# 按数据查找
def find_by_val(head, val):
    cur = head.next
    while cur:
        if cur.val == val:
            return cur
        cur = cur.next
    return None

# 按数据修改
def modify_by_val(head, old_val, new_val):
    cur = head.next
    while cur:
        if cur.val == old_val:
            cur.val = new_val
            return True
        cur = cur.next
    return False

# 清空
def clear(head):
    head.next = None

# 倒置(逆序)
def reverse(head):
    if head.next == None or head.next.next == None:
        return head
    pre = None
    cur = head.next
    while cur:
        tmp = cur.next
        cur.next = pre
        pre = cur
        cur = tmp
    head.next = pre
    return head

# 排序
def sort(head):
    if head.next == None or head.next.next == None:
        return head
    cur = head.next.next
    head.next.next = None
    while cur:
        pre = head
        while pre.next and pre.next.val < cur.val:
            pre = pre.next
        tmp = cur.next
        cur.next = pre.next
        pre.next = cur
        cur = tmp
    return head