Reverse Linked List

Reverse a singly linked list.

思路：

to slove this problem, I choose a recurrent method, to reverse linked-list nodes one-by-one.

first, point head to NULL.(make the head be a tail)

and then, point cur node to prev node, and then, move prev pointer to cur node, move cur pointer to next node.

until next node move to NULL.

*recursive(递归)，recurrent(递推)

/**
 * 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 *prev = head, *cur = head->next, *next = cur->next;
        head->next = NULL;
        while (next != NULL) {
            cur->next = prev;
            prev = cur;
            cur = next;
            next = next->next;
        }
        cur->next = prev;
        return cur;
    }
};