合并两个有序链表

• 方法1：迭代

  m、n为两个有序链表的长度

  时间复杂度：O(m+n)

  空间复杂度：O(1)

  class ListNode:
      def __init__(self, val=0, next=None):
          self.val = val
          self.next = next
  class Solution:
      def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:
          prehead = ListNode(-1)
          pre = prehead
          while l1 and l2:
              if l1.val <= l2.val:
                  pre.next = l1
                  l1 = l1.next
              else:
                  pre.next = l2
                  l2 = l2.next
              pre = pre.next
          pre.next = l1 if l1 is not None else l2
          return prehead.next
  

• 方法2：递归

  m、n为两个有序链表的长度

  时间复杂度：O(m+n)

  空间复杂度：O(m+n)

  class ListNode:
      def __init__(self,val=0,next=None):
          self.val = val
          self.next = next
  class Solution:
      def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:
          if not l1:
              return l2
          if not l2:
              return l1
          if l1.val <= l2.val:
              l1.next = self.mergeTwoLists(l1.next,l2)
              return l1
          else:
              l2.next = self.mergeTwoLists(l1,l2.next)
              return l2