21. 合并两个有序链表

来源: 21. 合并两个有序链表

1. 解题思路

递归或者非递归

2. 代码

2.1 递归

class Solution:
    def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:
        if not l1: return l2  # 终止条件，直到两个链表都空
        if not l2: return l1
        if l1.val <= l2.val:  # 递归调用
            l1.next = self.mergeTwoLists(l1.next,l2)
            return l1
        else:
            l2.next = self.mergeTwoLists(l1,l2.next)
            return l2

2.2 非递归

class Solution(object):
    def mergeTwoLists(self, l1, l2):
        head = ListNode(0)
        front = head
        while l1 or l2:
            if not l1:
                front.next = l2
                return head.next
            elif not l2:
                front.next = l1
                return head.next
            else:
                if l1.val>=l2.val:
                    front.next = l2
                    l2 = l2.next
                else:
                    front.next = l1
                    l1 = l1.next
            front = front.next
        return head.next