反转链表

反转一个单链表。

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？

 

迭代：

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

 

 

递归：

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