Reverse Linked List

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) {        if (head == NULL)        {            return NULL;        }        ListNode *p = NULL;        ListNode *q = head;        ListNode *r = NULL;        while (q != NULL)        {            r = q->next;            q->next = p;            p = q;            q = r;        }        return p;    }};