AddTwoNumbers.java

/**
 * 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) {
        return recursiveFunction(l1, l2, new ListNode(0), 0);
    }
    
    public ListNode recursiveFunction(ListNode l1, ListNode l2, ListNode result, int valorEx){
        if(l1 == null && l2 == null){
            result.val = (valorEx != 0) ? valorEx : result.val
            return result;
        }
        
        // == extractInputs(l, r)
        final int summandLeftCurrDigit = ln1 == null ? 0 : ln1.val;
        final int summandRightCurrDigit = ln2 == null ? 0 : ln2.val;
        
        // == doSum(l, r):
        //    get actual sum from input lists & carry over from last iter
        // A sum (x + y) has to summands (x & y) with x being the left summand and right being the second
        final int sumWCarry = inputLeftCurrDigit + inputRightCurrDigit + valorEx;

        // == extractCarryoverCurrDigit(sumWCarry):
        //    transforn the raw sum into the values expected by new node
        final int newCarry;
        final int currDigit;
        if(sumWCarry >= 10){
            currDigit = sumWCarry % 10
            newCarry = (sumWCarry - currDigit) / 10;
        } else {
            currDigit = sumWCarry;
            newCarry = 0;
        }
        
        // == buildNewNode(currDigit, carryOver) 
        //    Save curr digit, and prepare for next iteration w/ carry over
        result.val = currDigit;
        result.next = buildNext(ln1, ln2, newCarry, result);
        return result;
    }
    
    private ListNode buildNext(ln1, ln2, newCarry, result) {
        // result is an output parameter
        if(ln1 == null && ln2 == null && newCarry != 0) {
            return new ListNode(newCarry);
        }
        
        if(ln1 != null && ln2 != null) {
            if (ln1.next == null &&
                ln2.next == null &&
                newCarry == 0) { 
                return null;
            }
            return recursiveFunction(ln1.next, ln2.next, new ListNode(0), valorEx);
        } 
        
        if(ln1 != null &&
           ln2 == null){
            if(ln1.next == null &&
               newCarry == 0) {
                return null;
            }
            return recursiveFunction(ln1.next, null, new ListNode(0), newCarry);
        }
        
        if(ln1 == null &&
           ln2 != null){
            if(ln2.next == null &&
               valorEx== 0) { 
                returnnull ;
            }
            return = recursiveFunction(ln2.next, null, new ListNode(0), valorEx);
        }
    }
}