206. Reverse Linked List

Description

Reverse a singly linked list.

Hint:A linked list can be reversed either iteratively or recursively. Could you implement both?

Solution

Iterative

可以用head作为curr，这样可以省略一个局部变量。
另外prev初始为null是比较简洁的写法。

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

Recursive

递归的写法还蛮tricky的，重点是保证reverse后原链表的head指向reverse后的链表尾部，而不是指向null。举个例子：

• 原链表：1->2->3
• 一层递归后：1->2<-3
• 二层递归后：1<-2<-3

/**
 * 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;
    }
}