反转链表(简单)

反转链表

1、双指针

struct ListNode
{
    int val;
    ListNode* next;
    ListNode(int x):val(x), next(nullptr){}
};
class Solution{
    ListNode* reverseList(ListNode* head){
        ListNode* temp;
        ListNode* cur = head;
        ListNode* pre = NULL;
        while(cur)
        {
            temp = cur.next;
            cur.next = pre;
            pre = cur;
            cur = temp;
        }
        return pre;
    }
}

class ListNode:
    def __init__(self, val = 0; next = None):
        self.val = val
        self.next = None
class Solution:
    def reverseList(self, head):
        pre = None
        cur = head
        while(cur):
            temp = cur.next
            cur.next = pre
            pre = cur
            cur = temp
        return pre

2、递归法

struct ListNode{
    int val;
    ListNode* next;
    ListNode(x): val(x), next(nullptr){}
};
class Solution
{
    ListNode* reverse(ListNode*pre, ListNode*cur)
    {
        if(cur == NULL)
        {
            return pre;
        }
        ListNode*temp = cur.next;
        cur.next = pre;
        return reverse(cur, temp);
        
    }
    
    ListNode* reverseList(ListNode* head){
        reverse(NULL, head);
    }
}

class Solution:
    def reverse(self, pre, cur):
        if(cur == None):
            return pre;
        temp = cur.next
        cur.next = pre
        return self.reverse(cur, temp)
    def reverseList(self, head):
        return self.reverse(None, head)