Reverse a singly linked list.

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