Reverse Linked List

 

Reverse a singly linked list.

单链表的逆序有两种方法，一种是递归的，另一种是非递归的（头插法）。

递归解法如下，耗时11ms：

/**
 * 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 == NULL || head->next == NULL)
            return head;
        ListNode* p = reverseList(head->next);
        head->next->next = head;
        head->next = NULL;
        return p;
    }
};

非递归解法如下，耗时8ms：

/**
 * 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 == NULL || head->next == NULL)
            return head;
        ListNode* p = head->next;
        head->next = NULL;
        ListNode* q;
        while(p) {
            q = p;
            p = p->next;
            q->next = head;
            head = q;
        }
        return head;
    }
};