Problem description

You are given two non-empty linked lists representing two non-negative integers. 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.
You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Input & Output

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

Code

/**
 * 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) {
        
        ListNode res = new ListNode(0);
        ListNode p = l1;
        ListNode q = l2;
        
        ListNode curr = res;
        int flag = 0;
        
        while(q != null && p != null) {
            
            ListNode temp = new ListNode((p.val + q.val + flag) % 10);
            curr.next = temp;
            curr = temp;
            
            flag = (p.val + q.val + flag) / 10;
            q = q.next;
            p = p.next;
        }
        
        while(p != null) {
            ListNode temp = new ListNode((p.val + flag) % 10);            
            curr.next = temp;
            curr = temp;
            
            flag = (p.val + flag) / 10;
            p = p.next;
        }
        
        while(q != null) {
            ListNode temp = new ListNode((q.val + flag) % 10);
            curr.next = temp;
            curr = temp;
            
            flag = (q.val + flag) / 10;
            q = q.next;
        }
        
        if(flag != 0) {
            ListNode temp = new ListNode(flag);            
            curr.next = temp;
            curr = temp;
        }
        
        return res.next;
    }
}

Analysis

• 注意链表的建立过程。