ReverseIterativeLL.java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */

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

class Solution {
    private int len(ListNode head){
        ListNode temp = head;
        int counter = 0;
        while(temp!=null){
            counter++;
            temp = temp.next;
        }
        return counter;
}
    private ListNode getNode(int index, ListNode head){
        ListNode temp = head;
        for(int i = 1; i<=index; i++){
            temp = temp.next;
        }
        return temp;
    }
    public ListNode reverseList2(ListNode head) {
        int i = 0;
        int j = len(head)-1;
        while(i<j){
            ListNode leftNode = getNode(i, head);
            ListNode rightNode = getNode(j, head);
            int temp = leftNode.val;
            leftNode.val = rightNode.val;
            rightNode.val = temp;
            i++;
            j--;
        }
        return head;
    }
    
    public ListNode reverseList(ListNode head) {
        if(head == null){
return head;}
        // Maintain 3 Pointors
        ListNode prev = head;
        ListNode current = prev.next;
        while(current!=null){
            ListNode ahead = current.next;
            current.next = prev;
            prev = current;
            current = ahead;
        }
        ListNode temp = head;
        head = prev;
        temp.next= null;
        return head;
        
    }
}