LeetCode 206: 反转链表

[guodongxiaren]

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

反转一个单链表。

示例:

输入: 1->2->3->4->5->NULL
输出: 5->4->3->2->1->NULL

进阶: 你可以迭代或递归地反转链表。你能否用两种方法解决这道题？

[guodongxiaren]

迭代解法

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if (!head || !head->next) {
            return head;
        }
        ListNode* pre = NULL;
        ListNode* node = head;

        while (node) {
            ListNode* tmp = node->next;
            node->next = pre;
            pre = node;
            node = tmp;
        }
        return pre;
    }
};

[guodongxiaren]

递归解法

递归解法叹为观止！

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if (!head || !head->next) {
            return head;
        }
        ListNode* node = reverseList(head->next);

        head->next->next = head;
        head->next = NULL;
        return node;
    }
};