Leetcode: Reverse Linked List

Question

Reverse a singly linked list.

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Solution 1

Analysis

recursive method

Code

# Definition for singly-linked list.# class ListNode(object):#     def __init__(self, x):#         self.val = x#         self.next = Noneclass Solution(object):    def reverseList(self, head):        """        :type head: ListNode        :rtype: ListNode        """        if head==None or head.next==None:            return head        tail = head.next        n = self.reverseList(tail)        head.next = None        tail.next = head        return n

/** * 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* second = head->next;        ListNode* rest_head = reverseList(second);        second->next = head;        head->next = NULL;        return rest_head;    }};

Solution2

Analysis

iterative method
Example:

input:   1->2->3->4->Nonep  l 1  2->3->4->Nonep     l2->1  3->4->Nonep        l3->2->1  4->Nonep            l 4->3->2->1  None

Code

# Definition for singly-linked list.# class ListNode(object):#     def __init__(self, x):#         self.val = x#         self.next = Noneclass Solution(object):    def reverseList(self, head):        """        :type head: ListNode        :rtype: ListNode        """        if head==None or head.next==None:            return head        p = head;       # p is the head of reversed likedin list        l = head.next   # l is the head of the rest likedin list        p.next = None           while(l):            t = l.next  # after this, l is middle node between reversed list and the rest list            l.next = p            p = l            l = t        return p

/** * 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* p = head;        ListNode* l = head->next;        head->next = NULL;        while(l){            ListNode* t = l->next;            l->next = p;            p = l;            l = t;        }        return p;    }};

When thinking it in iterative way, we need to get what intermediate state looks like. For this problem, several nodes are reversed with header pointer, and the rest nodes with the header pointer.

Actually, recursive method reversed linked list one by one from the ending node to the beginning node. Iterative method does it from the starting node.

Iterative method can’t reverse node from the ending node. It is easy to know that additional time O(n) will be cost to obtain the previous node of p.