LeetCode：21. 合并两个有序链表

1、题目描述

将两个有序链表合并为一个新的有序链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的。 

示例：

　　输入：1->2->4, 1->3->4

　　输出：1->1->2->3->4->4

2、题解

2.1、解法一

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        if l1 == None:
            return l2
        if l2 == None:
            return l1
        
        left,right = l1,l2
        if left.val < right.val:
            new = left
            left = left.next
        else:
            new = right
            right = right.next

        node = new
        while left and right:
            if left.val <= right.val:
                node.next = left
                left = left.next
            else:
                node.next = right
                right = right.next
            node = node.next

        while left:
            node.next = left
            left = left.next
            node = node.next

        while right:
            node.next = right
            right = right.next
            node = node.next

        return new