合并两个有序链表 · 实现

class Node:
    def __init__(____):
        self.val = val
        self.next = next

def _to_csv(____):
    out = []
    while head:
        out.append(str(____))
        head = head.next
    return "____".join(____)

def merge(____):
    dummy = Node(____)
    cur = dummy
    while a and b:
        if a.val <= b.val:
            cur.next = a
            a = a.next
        else:
            cur.next = b
            b = b.next
        cur = cur.next
    cur.next = a or b
    return dummy.next

def solve():
    a = Node(____, Node(____, Node(____)))
    b = Node(____, Node(____))
    return _to_csv(merge(____))