reverse list

解法1：

/**
 * Definition of ListNode
 * 
 * class ListNode {
 * public:
 *     int val;
 *     ListNode *next;
 * 
 *     ListNode(int val) {
 *         this->val = val;
 *         this->next = NULL;
 *     }
 * }
 */
class Solution {
public:
    /**
     * @param head: The first node of linked list.
     * @return: The new head of reversed linked list.
     */
    ListNode *reverse(ListNode *head) {
        // write your code here
        ListNode* p = head;
        ListNode* newHead = NULL;
        while (p){
            
            ListNode* t = p->next;
            p->next = newHead;
            newHead = p;
            p = t;
        }
        return newHead;
    }
};

解法2：递归

/**
 * Definition of ListNode
 * 
 * class ListNode {
 * public:
 *     int val;
 *     ListNode *next;
 * 
 *     ListNode(int val) {
 *         this->val = val;
 *         this->next = NULL;
 *     }
 * }
 */
class Solution {
public:
    /**
     * @param head: The first node of linked list.
     * @return: The new head of reversed linked list.
     */
    ListNode *reverse(ListNode *head) {
        // write your code here
        if (head == NULL || head->next == NULL)
            return head;
        ListNode* newHead = reverse(head->next);
        head->next->next = head;
        head->next = NULL;
        return newHead;
    }
};