206. Reverse Linked List（python+cpp）

题目：

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?

解释：
单链表翻转。
循环写法，原地翻转：
python代码：

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def reverseList(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        #我觉得还是不要把head当做工作指针
        if not head:
            return None
        pre=None
        cur=head
        while cur:
            then=cur.next
            cur.next=pre
            pre=cur
            cur=then
        return pre        

c++代码：

/**
 * 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)
            return head;
        ListNode* pre=NULL;
        ListNode* cur=head;
        while(cur)
        {
            ListNode* then=cur->next;
            cur->next=pre;
            pre=cur;
            cur=then;
        }
        return pre; 
    }
};

递归写法：
python代码：

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def reverseList(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        if not head:
            return None
        if not head.next:
            return head
        p=head.next
        n=self.reverseList(p)
        head.next=None
        p.next=head
        return n       

c++代码：

/**
 * 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)
        {
            return NULL;
        }
        if (head->next==NULL)
        {
          return head;
        }
        ListNode* p=head->next;
        ListNode* n=reverseList(p);
        head->next=NULL;
        p->next=head;
        return n;
    }
};

总结：
单链表翻转是要原地翻转的，原地翻转的循环解法是最经典的，需要记住。