LeetCode Reverse Linked List

绝对老题了，感觉已经重复了

Reverse a singly linked list.

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Hint:

A linked list can be reversed either iteratively or recursively. Could you implement both?

递归版本

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
 // 19:22
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        return reverseList(NULL, head);
    }
    
    ListNode* reverseList(ListNode* last, ListNode* head) {
        if (head == NULL) {
            return last;
        }
        ListNode* rest = head->next;
        ListNode* nhead = reverseList(head, rest);
        head->next = last;
        return nhead;
    }
};

非递归版本

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
 // 19:20
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if (head == NULL) {
            return head;
        }
        ListNode* pre = NULL;
        ListNode* cur = head;
        while (cur != NULL) {
            ListNode* tmp = cur->next;
            cur->next = pre;
            pre = cur;
            cur = tmp;
        }
        return pre;
    }
};