Leetcode topic analysis add binary

Given two binary strings, return their sum (also a binary string).

For example,

a = "11"

b = "1"

Return "100".

    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        if (l1 == null && l2 == null) {
            return null;
        }
        if (l1 == null) {
            return l2;
        }
        if (l2 == null) {
            return l1;
        }
        ListNode p1 = l1;
        ListNode p2 = l2;
        int carry = 0;
        ListNode head = new ListNode(0);
        ListNode result = head;
        while (carry != 0 || p1 != null || p2 != null) {
            int v1 = 0;
            if (p1 != null) {
                v1 = p1.val;
                p1 = p1.next;
            }
            int v2 = 0;
            if (p2 != null) {
                v2 = p2.val;
                p2 = p2.next;
            }
            int tmp = v1 + v2 + carry;
            carry = tmp / 10;
            head.next = new ListNode(tmp % 10);
            head = head.next;
        }
        return result.next;
    }