单链表创建

def solve():
    class Node:
        def __init__(self, data):
            self.data = data
            self.next = None
    head = None
    for val in [1, 2, 3]:
        new_node = Node(val)
        if head is None:
            head = new_node
        else:
            cur = head
            while cur.next:
                cur = cur.next
            cur.next = new_node
    cur = head
    while cur:
        print(cur.data)
        cur = cur.next