LeetCode第六天

21. Merge Two Sorted Lists

这道题类似于中等难度的第一道题，所以写起来就比较容易。先生成一个头结点，初始化一下。因为这两个List已经排好序了，所以先当两个List都不空时，逐个判断，添加到新List即可，记得维护指针，往后顺延。跳出循环肯定是有一个List到头了，所以在之后再判断一下就行了。

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

class Solution:
    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        res = ListNode(0)
        result = res
        while l1 and l2:
            if l1.val < l2.val:
                res.next = ListNode(l1.val)
                l1 = l1.next
                res = res.next
            elif l1.val == l2.val:
                res.next = ListNode(l1.val)
                res = res.next
                res.next = ListNode(l2.val)
                res = res.next
                l1 = l1.next
                l2 = l2.next
            else:
                res.next = ListNode(l2.val)
                l2 = l2.next
                res = res.next
        while l1:
            res.next = ListNode(l1.val)
            res = res.next
            l1 = l1.next
        while l2:
            res.next = ListNode(l2.val)
            res = res.next
            l2 = l2.next
        return result.next