［LeetCode］206. Reverse Linked List 解题小结

题目：

Reverse a singly linked list.

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Hint:

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

用循环来做

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */

/**
 * Reverse Iteratively
 */
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        ListNode* revHead = NULL;
        ListNode* rev = head;

        while(rev){
            head = rev;
            rev = rev->next;
            head->next = revHead;
            revHead = head;
        }

        return revHead;

    }
};

用递归来做：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */

/**
 * Reverse Iteratively
 */
class Solution {
public:
    ListNode* reverseList(ListNode* head) {

        if(!head || (!head->next))return head;
        ListNode* node = reverseList(head->next);
        head->next->next = head;
        head->next = NULL;
        return node;

    }
};