链表实现两个数字相加

两个用链表表示的数字相加
https://leetcode.com/problems/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
从头开始加，若后面不够需要新建链表结点。
若想方便可以不断新建结点保存每位的和，若要时间复杂度底，就直接加在某个链表上。
此处演示加在某个链表上的代码

public ListNode addTwoNumbers(ListNode l1, ListNode l2){    if(l1 == null) return l2;    else if(l2 == null) return l1;    else{        ListNode ls1 = l1, ls2 = l2;        int sum = 0, prog = 0;        while(ls1 != null && ls2 != null){            sum = ls1.val + ls2.val+ prog;            ls1.val = sum%10;            prog = sum/10;            if(ls1.next == null || ls2.next == null) break;            ls1 = ls1.next;            ls2 = ls2.next;        }        if(ls1.next == null) ls1.next = ls2.next;        while(prog != 0){            if(ls1.next == null) {                ls1.next = new ListNode(prog);                prog = 0;            }            else{                ls1 = ls1.next;                sum = ls1.val + prog;                ls1.val = sum%10;                prog = sum/10;            }        }    }    return l1;}