206反转链表_Leon

反转一个单链表。

示例:

输入: 1->2->3->4->5->NULL
输出: 5->4->3->2->1->NULL

思路：用栈实现：

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

class Solution:
    def reverseList(self, head: ListNode) -> ListNode:
        stack = []
        res_Head = ListNode(0)
        cur = res_Head
        cnt = 0
        while(head):
            cnt += 1
            stack.append(head.val)
            head = head.next
        while(cnt):
            cur.next = ListNode(stack.pop())
            cur = cur.next
            cnt -= 1
        return res_Head.next
            

递归实现:

class Solution:
    def reverseList(self, head: ListNode) -> ListNode:
        if(head == None or head.next == None):
            return head
        p = self.reverseList(head.next)
        head.next.next = head
        head.next = None
        return p