2. Add Two Numbers

class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode preNode = new ListNode(0);
        ListNode tail = preNode;
        int pre = 0;
        while (l1!=null&&l2!=null){
            int val1 = l1.val;
            int val2 = l2.val;
            int res = val1+val2+pre;
            pre = res/10;
            tail.next = new ListNode(res%10);
            tail=tail.next;
            l1=l1.next;
            l2=l2.next;
        }
        while (l1!=null){
            tail.next = new ListNode((l1.val+pre)%10);
            tail=tail.next;
            pre = (l1.val+pre)/10;
            l1=l1.next;

        }
        while (l2!=null){
            tail.next = new ListNode((l2.val+pre)%10);
            tail=tail.next;
             pre = (l2.val+pre)/10;
            l2= l2.next;

        }
        if (pre!=0)tail.next = new ListNode(pre);
        return preNode.next;
    }
}