206. Reverse Linked List 反转链表

206. Reverse Linked List 

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Reverse a singly linked list.

Hint:

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

 

/**
 * 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* pReversedHead = nullptr;
        ListNode* pNode = head;
        ListNode* pPrev = nullptr;
        while(pNode != nullptr)  
        {
            ListNode* pNext = pNode->next; //record pNext
            if (pNext == nullptr) // pReversedHead
                pReversedHead = pNode; 
            pNode->next = pPrev; // reverse
            pPrev = pNode;  
            pNode = pNext;
        }
        return pReversedHead;
    }
};