Add Two Numbers - https://leetcode.com/problems/add-two-numbers/description/

gistfile1.txt

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
      // Cases"
      // 1. Same size
      // 2. Diff length
      // 3. Either null
      // 4. No leading zero
      
      if (l1 == null || l2 == null) {
        return null;
      }
      
      ListNode curL1 = l1;
      ListNode curL2 = l2;
      
      final ListNode resultRoot = new ListNode(0);
      
      int carry = 0;
      
      ListNode curResult = resultRoot;
      while (curL1 != null || curL2 != null) {
        
        int valL1 = curL1 != null ? curL1.val : 0;
        int valL2 = curL2 != null ? curL2.val : 0;
        
        // operations
        int sum = carry + valL1 + valL2;
        int reminder = sum % 10;
        carry = sum / 10;
        
        // update result
        curResult.next = new ListNode(reminder);
        
        // Update pointers
        curL1 = curL1 != null ? curL1.next : null;
        curL2 = curL2 != null ? curL2.next : null;
        curResult = curResult.next;
        
      }
      
      // Check carry
      if (carry > 0) {
        curResult.next = new ListNode(carry);
      }
      
      return resultRoot.next;
      
    }
}

OR

public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
    ListNode dummyHead = new ListNode(0);
    ListNode p = l1, q = l2, curr = dummyHead;
    int carry = 0;
    while (p != null || q != null) {
        int x = (p != null) ? p.val : 0;
        int y = (q != null) ? q.val : 0;
        int sum = carry + x + y;
        carry = sum / 10;
        curr.next = new ListNode(sum % 10);
        curr = curr.next;
        if (p != null) p = p.next;
        if (q != null) q = q.next;
    }
    if (carry > 0) {
        curr.next = new ListNode(carry);
    }
    return dummyHead.next;
}