反转单链表 · 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 reverse(____):
    prev = None
    cur = head
    while cur:
        nxt = cur.next
        cur.next = prev
        prev = cur
        cur = nxt
    return prev

def solve():
    return _to_csv(reverse(Node(____, Node(____, Node(____)))))