4. Add Two Numbers

You are given two linked lists representing two non-negative numbers.

The digits are stored in reverse order and each of their nodes contain a single digit.

Add the two numbers and return it as a linked list.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

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Solution1: iterative

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        
        ListNode rstHead = new ListNode(0);
        ListNode rst = rstHead;
        int carry = 0;
        
        while(l1 != null || l2 != null || carry != 0){
            
            int val = carry;
            if(l1!=null) val+=l1.val;
            if(l2!=null) val+=l2.val;
            
            ListNode node = new ListNode(val % 10);
            
            rst.next = node;
            rst = rst.next;
            
            carry = val / 10;
            if(l1!=null) l1 = l1.next;
            if(l2!=null) l2 = l2.next;
            
        }
        
        return rstHead.next;
        
    }
}

 

Solution2: Recursive

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        
        return helper(l1, l2, 0);
        
    }
    
    public ListNode helper(ListNode n1, ListNode n2, int carry) {

        if (n1 == null && n2 == null && carry == 0)
            return null;

        ListNode node;
        int val = carry;
        
        if (n1 != null)
            val += n1.val;
        if (n2 != null)
            val += n2.val;        
        node = new ListNode(val % 10);
        
        // recursive
        if(n1 != null || n2 != null || val > 10){
            if(n1 != null)
                n1 = n1.next;
            if(n2 != null)
                n2 = n2.next;
            ListNode nextNode = helper(n1,n2, val/10);
            node.next = nextNode;
        }

        return node;
    }
}