Reverse Linked List（反转单向链表）

来源：https://leetcode.com/problems/reverse-linked-list

Reverse a singly linked list.

递归方法：递归调用直到最后一个节点再开始反转，注意保存反转后的头结点返回

Java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
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;
    }
}

Python

# -*- coding:utf-8 -*-
# 递归
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
class Solution:
    # 返回ListNode
    def ReverseList(self, pHead):
        # write code here
        if pHead == None or pHead.next == None:
            return pHead
        tmp = self.ReverseList(pHead.next)
        pHead.next.next = pHead
        pHead.next = None
        return tmp

迭代方法：两个指针从头开始依次反转，注意保存下一次反转的节点指针

Java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        if(head == null || head.next == null) {
            return head;
        }
        ListNode pre = head, cur = head.next, next = null;
        while(cur != null) {
            next = cur.next;
            cur.next = pre;
            if(pre == head) {
                pre.next = null;
            }
            pre = cur;
            cur = next;
        }
        return pre;
    }
}

Python

# -*- coding:utf-8 -*-
# 迭代
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
class Solution:
    # 返回ListNode
    def ReverseList(self, pHead):
        # write code here
        if pHead == None or pHead.next == None:
            return pHead
        pre_node, cur_node = pHead, pHead.next
        while pre_node and cur_node:
            tmp_node = cur_node.next
            cur_node.next = pre_node
            pre_node = cur_node
            cur_node = tmp_node
        pHead.next = None
        return pre_node