【重点】2.两数相加

题目

class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        if (l1 == null) {
            return l2;
        }
        if (l2 == null) {
            return l1;
        }
        ListNode head = null, cur = null;
        int carry = 0; // 进位
        while (l1 != null || l2 != null) {
            if (head == null) {
                int tmp = l1.val + l2.val;
                head = new ListNode(tmp % 10);
                cur = head;
                carry = tmp / 10;
                l1 = l1.next;
                l2 = l2.next;
            } else {
                int tmp = carry;
                if (l1 != null) {
                    tmp += l1.val;
                    l1 = l1.next;
                } 
                if (l2 != null) {
                    tmp += l2.val;
                    l2 = l2.next;
                }
                cur.next = new ListNode(tmp % 10);
                carry = tmp / 10;
                cur = cur.next;
            }
        }
        if (carry > 0) {
            cur.next = new ListNode(carry);
        }

        return head;
    }
}