206、Reverse Linked List

Reverse a singly linked list.

Example:

Input: 1->2->3->4->5->NULL
Output: 5->4->3->2->1->NULL

Follow up:

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

思路

经典的链表逆序问题，题目要求两种方式实现：迭代和非迭代

c++11

非递归方法

定义一个新的头指针为空，之后遍历原链表，将head插入到new_head前面，同时更新new_head

/**
 * 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) {
        ListNode* new_head = NULL;
        while (head) {
            ListNode* tmp = head->next;
            head->next = new_head;
            new_head = head;
            head = tmp;
        }
        return new_head;
    }
};

ac结果：

Runtime: 8 ms, faster than 100.00% of C++ online submissions for Reverse Linked List.
Memory Usage: 9.3 MB, less than 19.73% of C++ online submissions for Reverse Linked List.

递归方法