leetcode 206. Reverse Linked List

Reverse a singly linked list.

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Hint:

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

遞歸:

/**
 * 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 == NULL || head->next == NULL) return head;
        ListNode* newhead = reverseList(head->next);
        head->next->next = head;
        head->next = NULL;
        return newhead;
    }
};

循環：

/**
 * 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 == NULL || head->next == NULL) return head;
        ListNode* newhead = NULL;
        ListNode* p = head;
        while(p){
            ListNode* tmp = p;
            p = p->next;
            tmp->next = newhead;
            newhead = tmp;
        }
        return newhead;
    }
};