类型：链表
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- 合并两个有序链表 💚
https://leetcode-cn.com/problems/merge-two-sorted-lists/
❓ 将两个升序链表合并为一个新的 升序 链表并返回。
💡 递归
class Solution: def mergeTwoLists(self, l1, l2): if l1 is None: return l2 elif l2 is None: return l1 elif l1.val < l2.val: l1.next = self.mergeTwoLists(l1.next, l2) return l1 else: l2.next = self.mergeTwoLists(l1, l2.next) return l2时间复杂度：O(m + n)，空间复杂度：O(m + n)
💡 迭代
class Solution: def mergeTwoLists(self, l1, l2): prehead = ListNode(-1) prev = prehead while l1 and l2: if l1.val <= l2.val: prev.next = l1 l1 = l1.next else: prev.next = l2 l2 = l2.next prev = prev.next # 合并后 l1 和 l2 最多只有一个还未被合并完，我们直接将链表末尾指向未合并完的链表即可 prev.next = l1 if l1 is not None else l2 return prehead.next时间复杂度：O(m + n)，空间复杂度：O(1)