LeetCode 206. 反转链表

206. Reverse Linked List

Given the head of a singly linked list, reverse the list, and return the reversed list.

Example 1:

Input: head = [1,2,3,4,5]

Output: [5,4,3,2,1]

Example 2:

Input: head = [1,2]

Output: [2,1]

Example 3:

Input: head = []

Output: []

解题思路：

1. 递归
2. 双指针

法一：

class Solution {
    public ListNode reverseList(ListNode head) {
        // Recursion 
        // Time: O(n) 
        // Space: O(n)
        if (head == null || head.next == null) {
            return head;
        }
        ListNode ret = reverseList(head.next);
        head.next.next = head;
        head.next = null;
        return ret;
    }
}

 法二：

class Solution {
    public ListNode reverseList(ListNode head) {
        // double pointer
        // Time: O(n)
        // Space: O(1)
        ListNode cur = head;
        ListNode pre = null;
        while (cur != null) {
            ListNode tempNode = cur.next;
            cur.next = pre;
            pre = cur;
            cur = tempNode;
        }
        return pre;
    }
}