leetcode Question: Reverse Linked List

Reverse Linked List

Reverse a singly linked list.

Analysis:

Classic problem. There are multiple algorithms to reverse the linked list. Here in this post I just describe the very basics. Some advanced algorithms can be found in other problems in my blog.

To illustrate the process, say we have the linked list:
  1->2->3->4->5->null
head

In order to reverse it, we define a new pointer:
newhead->null

Iterate the original list, each time insert the node to the start of the new linked list:
1. newhead->1->null
2. newhead->2->1->null
...
5 newhead->5->4->3->2->1->null

Then just return newhead->next.

Code(C++):

/**
 * 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 * newhead = new ListNode(0);
        while (head){
            ListNode * tmp = newhead->next;
            newhead->next = head;
            head = head->next;
            newhead->next->next = tmp;
        }
        return newhead->next;
        
    }
};

Code(Python):

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    # @param {ListNode} head
    # @return {ListNode}
    def reverseList(self, head):
        newhead = ListNode(0)
        while head:
            tmp = newhead.next
            newhead.next = head
            head = head.next
            newhead.next.next = tmp
        return newhead.next