leetcode 206. 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?

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;
ListNode* newhead = reverseList(head->next);
head->next->next = head;
head->next = NULL;
return newhead;
}
};

循环：
/**
* 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* newhead = NULL;
ListNode* p = head;
while(p){
ListNode* tmp = p;
p = p->next;
tmp->next = newhead;
newhead = tmp;
}
return newhead;
}
};