编程题目：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也要满足单调递增。

其实这个就是合并排序的变形，只是这里使用的是链表罢了，原理是一样的。

这里链表其实可以再加入一个尾节点，实现快速的append操作 0（1）。因为时间问题，这里就不做添加。

#!/usr/bin/env python3

class Node(object):
        def __init__(self, elem, next_=None):
                self.elem = elem
                self.next = next_

class List(object):
        def __init__(self):
                self.head = None
                self.num = 0

        def is_empty(self):
                return self.head is None

        def prepend(self, elem):
                self.head = Node(elem, self.head)
                self.num += 1

        def prepop(self):
                if self.is_empty():
                        raise ValueError("List is Empty")
                e = self.head.elem
                self.head = self.head.next
                self.num -= 1
                return e

        def append(self, elem):
                if self.is_empty():
                        self.head = Node(elem)
                        return
                p = self.head
                while p.next:
                        p = p.next
                p.next = Node(elem)
                self.num += 1

        def bianli(self):
                p = self.head
                li = []
                while p:
                        li.append(p.elem)
                        p = p.next
                return li

def merge_list(lista,listb):
        indexa = lista.head
        indexb = listb.head
        ml = List()
        while indexa and indexb:
                if indexa.elem <= indexb.elem:
                        ml.append(indexa.elem)
                        indexa = indexa.next
                else:
                        ml.append(indexb.elem)
                        indexb = indexb.next
        while indexa:
                ml.append(indexa.elem)
                indexa = indexa.next
        while indexb:
                ml.append(indexb.elem)
                indexb = indexb.next
        return ml.bianli()

if __name__ == "__main__":
        lista = List()
        for i in range(10,-1,-2):
                lista.prepend(i)
        print(lista.bianli())
        listb = List()
        for i in range(21,0,-2):
                listb.prepend(i)
        print(listb.bianli())
        print(merge_list(lista,listb))