leetcode 206. Reverse Linked List

Reverse a singly linked list.

思路：递归，每次保存一下上一个节点用来构造反转后的链表。
非递归，需要多记录两个值.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* ans = nullptr;
    void dfs(ListNode* p, ListNode* q) {
        if (p == nullptr) {
            ans = q;
            return ;
        }
        dfs(p->next, p);
        p->next = q;
        if (q) q->next = nullptr;
    } 
    ListNode* reverseList(ListNode* head) {
        dfs(head, nullptr);
        return ans;
    }
};

/**
 * 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 *prev = NULL;
        ListNode *next = NULL;

        while (head) {
            next = head->next;
            head->next = prev;
            prev = head;
            head = next;
        }
        return prev;
    }
};