Leetcode 21. 合并两个有序链表(Python3)

21. 合并两个有序链表

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

 

1.递归版

class Solution(object):
    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        if not l1:return l2
        if not l2:return l1
        res = ListNode(0)
        if l1.val >= l2.val:
            res = l2
            res.next = self.mergeTwoLists(l1,l2.next)
        else:
            res = l1
            res.next = self.mergeTwoLists(l1.next,l2)
        return res

 

2.循环版

class Solution(object):
    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        if not l1:return l2
        if not l2:return l1
        head = ListNode(0)
        node = head
        while l1 and l2:
            if l1.val <= l2.val:
                node.next,l1= l1,l1.next
            else:
                node.next,l2 = l2,l2.next
            node = node.next
        if l1:node.next = l1
        if l2:node.next = l2
        return head.next