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 || !(head->next)) return head;        ListNode* p=head->next;        ListNode* ln=reverseList(head->next);        p->next=head;        head->next=NULL;        return ln;    }};

之前错误：

 把p->next=head; 写成 ln->next=head;

ln是返回链表的头指针