合并两个有序链表

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():
    return '1,1,2,3,4'