Leetcode 206. Reverse Linked Lis

文章作者：Tyan
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1. Description

Reverse Linked List

2. Solution

• Iteratively

/**
 * 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* pre = nullptr;
        ListNode* next = nullptr;
        ListNode* current = head;
        while(current) {
            next = current->next;
            current->next = pre;
            pre = current;
            current = next;
        }
        return pre;
    }
};

• Recursively

/**
 * 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) {
        return reverse(head, nullptr);        
    }
    
private:
    ListNode* reverse(ListNode* current, ListNode* pre) {
        if(!current) {
            return nullptr;
        }
        ListNode* next = current->next;
        if(next) {
            current->next = pre;
            return reverse(next, current);
        }
        current->next = pre;
        return current;
    }
    
};

Reference

1. https://leetcode.com/problems/reverse-linked-list/description/