206. Reverse Linked List

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Author: hozhangel

Time:    2018-08-11  20:31:01

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206. Reverse Linked List

problem description:

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?

 My hint:

just one traverse is enough:

step 1: pre->1->2->3->4->5->NULL

            ||p   q

step 2: pre->2->1->3->4->5->NULL

                           q  ||p

step 3: pre->3->2->1->4->5->NULL

                               q   ||p

at every step || (skip to the next node),just move the current node to the pre->next,

  My code:

/**
 * 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 == nullptr || head->next == nullptr) {
            return head;
        }
        ListNode* pre = new ListNode(0);
        pre->next = head;
        ListNode* p = pre->next;
        ListNode* q = head->next;
        while (q) {
            
            ListNode* pt = q;
            p->next = pt->next;
            pt->next = pre->next;
            pre->next = pt;
            q=p->next;
        }

        return pre->next;
    }
};