单链表的增删改查代码

单链表的基本增删改查操作可以用不同的编程语言实现，这里以Python为例：

1. **创建/插入节点**：

class Node:
    def __init__(self, data=None):
        self.data = data
        self.next = None

def append(head, data):
    if not head:
        return Node(data)
    curr = head
    while curr.next:
        curr = curr.next
    curr.next = Node(data)
    return head

2. **查找/删除节点**：

def search_and_delete(head, target):
    if not head:
        return None
    if head.data == target:
        return head.next
    curr = head
    while curr.next and curr.next.data != target:
        curr = curr.next
    if curr.next is None:
        return head
    else:
        curr.next = curr.next.next
        return head

3. **修改节点**：

def update_node_value(head, key, new_data):
    curr = head
    while curr:
        if curr.data == key:
            curr.data = new_data
            break
        curr = curr.next
    return head

4. **查询节点个数**：

def count_nodes(head):
    count = 0
    curr = head
    while curr:
        count += 1
        curr = curr.next
    return count