[LintCode] Reverse Linked List 倒置链表

Reverse a linked list.

Have you met this question in a real interview? 

Yes

Example

For linked list 1->2->3, the reversed linked list is 3->2->1

Challenge 

Reverse it in-place and in one-pass

LeetCode上的原题，请参见我之前的博客Reverse Linked List。

解法一：

class Solution {
public:
    /**
     * @param head: The first node of linked list.
     * @return: The new head of reversed linked list.
     */
    ListNode *reverse(ListNode *head) {
        if (!head) return NULL;
        ListNode *dummy = new ListNode(-1), *cur = head;
        dummy->next = head;
        while (cur->next) {
            ListNode *t = cur->next;
            cur->next = t->next;
            t->next = dummy->next;
            dummy->next = t;
        }
        return dummy->next;
    }
};

解法二：

class Solution {
public:
    /**
     * @param head: The first node of linked list.
     * @return: The new head of reversed linked list.
     */
    ListNode *reverse(ListNode *head) {
        if (!head || !head->next) return head;
        ListNode *t = head;
        head = reverse(t->next);
        t->next->next = t;
        t->next = NULL;
        return head;
    }
};