leetcode解题: Add Two Numbers (2)

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

解法1：O(N + M) Time + O(1) Space

这题的坑是leetocde的OJ可能会TLE，要用tail.next = new ListNode(temp)就可以过了。
Java

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