兩個用連結串列表示的數字相加
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
從頭開始加，若後面不夠需要新建連結串列結點。
若想方便可以不斷新建結點儲存每位的和，若要時間複雜度底，就直接加在某個連結串列上。
此處演示加在某個連結串列上的程式碼

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;
}