力扣 206. 反转链表

前置

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

递归

def reverseList(head):
    if not head or not head.next: return head
    end = reverseList(head.next)
    head.next.next = head
    head.next = None
    return end

迭代 1

def reverseList(head):
    if not head or not head.next: return head
    pre, cur, nex = None, head, head.next
    while nex:
        cur.next = pre
        pre, cur, nex = cur, nex, nex.next
    cur.next = pre
    return cur

迭代 2

def reverseList(head):
    if not head or not head.next: return head
    pre, cur, nex = None, head, head.next
    while cur:
        cur.next = pre
        pre, cur = cur, nex
        if nex: nex = nex.next
    return pre