22.leetcode题目206： Reverse Linked List

题目：

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：
翻转链表。

根据提示：方法一，依次翻转

class Solution {
public:
ListNode* reverseList(ListNode* head) {
<span style="color:#006600;"> //reversed iteratively</span><span style="color:#ffff00;">
</span> if(head==NULL){
return head;
}
ListNode* res=head;
ListNode* p=res->next;
res->next=NULL;
while(p!=NULL){
ListNode* q=p->next;
p->next=res;
res=p;
p=q;
}
return res;
}
};方法二，递归调用
class Solution {
public:
ListNode* reverseList(ListNode* head) {
<span style="color:#006600;">//reversed recursively</span>
if(head==NULL||head->next==NULL){
return head;
}
ListNode* p=head->next;
ListNode* res=reverseList(p);
head->next=NULL;
p->next=head;
return res;
}
};