Reverse Linked List****

Reverse a singly linked list.

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Hint:

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

分析：p = p->next的位置很重要！！继续思考！

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

Runtime: 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 || !head->next) return head;
        
        ListNode* pre = head;
        ListNode* current = pre->next;
        pre->next = NULL;
        while(current){
            ListNode* post = current->next;
            current->next = pre;
            pre = current;
            current = post;
        }
        return pre;
    }
};