206. Reverse Linked List 链表

题目地址

头插法。

/** * 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 *ans, *pre;        ans->next = NULL;        while (head != NULL) {            ListNode *node = new ListNode(head->val);            node->next = ans->next;            ans->next = node;            pre = head;            head = head->next;            delete pre;        }        return ans->next;    }};