struct ListNode* ReverseList(struct ListNode* pHead ) {
    // write code here
    struct ListNode* p = pHead;
    struct ListNode* r ;
    struct ListNode* new = NULL;
    while(p!=NULL)
    {
        r=p->next;
        p->next = new;
        new = p;
        p=r;
    }
    return new; 
}