LeetCode 2. Add Two Numbers

add_two_numbers.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; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode c1 = null;
        ListNode c2 = null;
        ListNode s = null;
        ListNode p = null;
        ListNode n = null;
        int overflow = 0;
        
        c1 = l1;
        c2 = l2;
        
        while(c1 != null || c2 != null){
            int sum =0;
            if( c1 == null ){
                sum = c2.val + overflow;
            }else if( c2 == null){
                sum = c1.val + overflow;
            }else{
                sum = c1.val + c2.val + overflow;    
            }
                
            
            if( sum > 9 ){
                n = new ListNode(sum-10);
                overflow = 1;
            }else{
                n = new ListNode(sum);
                overflow = 0;
            }
            
            if( p != null ){
                p.next = n;
            }
            p = n;
            
            if( s == null ){
                s = p;
            }
            
            c1 = (c1 == null || c1.next == null)? null: c1.next;
            c2 = (c2 == null || c2.next == null)? null: c2.next;;
        }
        
        if(overflow > 0){
            n = new ListNode(1);
            p.next = n;
        }
            
        
        return s;
            
            
    }
}