Leetcode 206. Reverse Linked List

Similar Questions 

Reverse Linked List II Binary Tree Upside Down Palindrome Linked List

思路：链表反转。

解法一：迭代。

添加头节点（推荐）：不断将当前元素start插入dummy和dummy.next之间，实现反转。

/**
 * 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) return head;
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode pre = head, start = head.next;
        while(start != null) {
            pre.next = start.next;
            start.next = dummy.next;
            dummy.next = start;
            start = pre.next;
        }
        return dummy.next;
    }
}

不添加头节点：

/**
 * 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 p0, p1;
        p0 = null;
        p1 = head;
        while(p1 != null) {//确保p1不为空
            ListNode p2 = p1.next;
            p1.next = p0;
            p0 = p1;
            p1 = p2;
        }
        return p0;
    }
}

解法二：递归。

/**
 * 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;//这个判断很重要
        ListNode p1 = head.next;
        ListNode p0 = reverseList(p1);
        p1.next = head;
        head.next = null;
        return p0;
    }
}

Next challenges: Reverse Linked List II Binary Tree Upside Down