Description
Linked List
Python
Recursive solution:
# Definition for singly-linked list.
# class ListNode:
# def __init__(self, val=0, next=None):
# self.val = val
# self.next = next
class Solution:
def reverseList(self, head: ListNode) -> ListNode:
new_head = None
cur = head
while cur != None:
next_cur = cur.next
cur.next = new_head
new_head = cur
cur = next_cur
return new_head
Iterative solution:
比较难理解的方式,关键在于假设cur后面的已经都逆转了,现在这步该如何走
class Solution:
def reverseList(self, head: ListNode) -> ListNode:
if not head or not head.next:
return head
new_head = self.reverseList(head.next)
head.next.next = head
head.next = None
return new_head
Java
class Solution {
public ListNode reverseList(ListNode head) {
ListNode prev = null;
ListNode curr = head;
while (curr != null) {
ListNode next = curr.next;
curr.next = prev;
prev = curr;
curr = next;
}
return prev;
}
}
class Solution {
public ListNode reverseList(ListNode head) {
if (head == null || head.next == null) {
return head;
}
ListNode newHead = reverseList(head.next);
head.next.next = head;
head.next = null;
return newHead;
}
}