单链表求长度

def solve():
    class Node:
        def __init__(self, data):
            self.data = data
            self.next = None
    head = None
    for val in [5, 10, 15, 20]:
        new_node = Node(val)
        if head is None:
            head = new_node
        else:
            cur = head
            while cur.next:
                cur = cur.next
            cur.next = new_node
    length = 0
    cur = head
    while cur:
        length += 1
        cur = cur.next
    print(length)