反转单链表

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

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

def solve():
    h = Node(1, Node(2, Node(3)))
    vals = []
    while h:
        vals.append(str(h.val))
        h = h.next
    return ','.join(reversed(vals))