2019-05-20LeetCode21. 合并两个有序链表

归并的思路，就是卡在了循环没跳出来，及时注释，发现循环指针没有指向下一个 10min

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

class Solution:
    def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:
        head=ListNode(0)
        cur=head
        while(l1!=None and l2!=None):
            if(l1.val<=l2.val):
                cur.next=l1
                cur=cur.next  # 这里可以和下面合并
                l1=l1.next
            else:
                cur.next=l2
                cur=cur.next
                l2=l2.next
        while(l1!=None):  # cur.next = l1 or l2 可以用这个替换，python的魔法方法
            cur.next=l1
            cur=cur.next
            l1=l1.next
        while(l2!=None):
            cur.next=l2
            cur=cur.next
            l2=l2.next
        return head.next     

自己写的递归版本

class Solution:
    def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:
        # if not l1 or not l2: #  Or的巧妙
        #     return l1 or l2
        if l1==None and l2==None:
            return None
        if l1==None: return l2
        if l2==None: 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