(Easy) Reverse linked list LeetCode

Description:

Reverse a singly linked list.

Example:

Input: 1->2->3->4->5->NULL
Output: 5->4->3->2->1->NULL

Follow up:

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

Accepted

661,364

Submissions

1,172,734

 

Solution:

 

　　Explanation:

Iterative Method

1. Initialize three pointers prev as NULL, curr as head and next as NULL.
2. Iterate trough the linked list. In loop, do following.
   // Before changing next of current,
   // store next node
   next = curr->next

   // Now change next of current
   // This is where actual reversing happens
   curr->next = prev

   // Move prev and curr one step forward
   prev = curr
   curr = next

 

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        
        ListNode prev =null, curr = head, next = null;
        
        
        while(curr!=null){
            
            
             next = curr.next; 
            
             curr.next = prev;
            
             prev = curr; 
            
             curr = next;
        }
        
        
        return prev;
        
        
    }
}