合并两个排序的链表

题目描述
输入两个单调递增的链表，输出两个链表合成后的链表，当然我们需要合成后的链表满足单调不减规则。
非递归方法：

def Merge(self, l1, l2):
    dummy = cur = ListNode(0)
    while l1 and l2:
        if l1.val < l2.val:
            cur.next = l1
            l1 = l1.next
        else:
            cur.next = l2
            l2 = l2.next
        cur = cur.next
    cur.next = l1 or l2
    return dummy.next

递归方法：

def Merge(self, l1, l2):
    if not l1 or not l2:
        return l1 or l2
    if l1.val < l2.val:
        l1.next = self.Merge(l1.next, l2)
        return l1
    else:
        l2.next = self.Merge(l1, l2.next)
        return l2

递归方法优化版：

def Merge(self, pHead1, pHead2):
    # write code here
    if pHead1 and pHead2:
        if pHead1.val > pHead2.val:
            pHead1, pHead2 = pHead2, pHead1
        pHead1.next = self.Merge(pHead1.next, pHead2)
    return pHead1 or pHead2