206 Reverse 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) {
        
        if(head==NULL||head->next==NULL)
            return head;
        ListNode* pre = NULL,*mid = head,*last = head->next;
        while(last)
        {
            mid->next = pre;
            pre = mid;
            mid = last;
            last = last->next;
        }
        mid->next = pre;
        return mid;
    }
};