219. 在排序链表中插入一个节点

样例 1：

输入：head = 1->4->6->8->null, val = 5
输出：1->4->5->6->8->null

样例 2：

输入：head = 1->null, val = 2
输出：1->2->null
第一个版本:

"""
Definition of ListNode
class ListNode(object):
    def __init__(self, val, next=None):
        self.val = val
        self.next = next
"""

class Solution:
    """
    @param head: The head of linked list.
    @param val: An integer.
    @return: The head of new linked list.
    """
    def insertNode(self, head, val):
        # write your code here
        new_node = ListNode(val)
        #边界情况
        if not head: return None
        if head.val >= val: 
            new_node.next = head 
            return new_node
        
        p = head
        
        while p.next:
            #如果在中间的话，则直接返回
            if val > p.val and val <= p.next.val:
                new_node.next = p.next
                p.next = new_node
                return head
            else:
                p = p.next 
        else:
            #如果不在中间
            p.next = new_node
        
        return head

第二个版本:

"""
Definition of ListNode
class ListNode(object):
    def __init__(self, val, next=None):
        self.val = val
        self.next = next
"""

class Solution:
    """
    @param head: The head of linked list.
    @param val: An integer.
    @return: The head of new linked list.
    """
    def insertNode(self, head, val):
        # write your code here
        #一直到val < p.next.val 的时候退出
        dummy = ListNode(0)
        dummy.next = head 
        
        p = dummy 
        while p.next  and val < p.next.val:
            p = p.next
        
        #开始拼接
        new_node = ListNode(val, p.next)
        p.next = new_node
        
        return head