2. 两数相加.java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode head = new ListNode(0);
        ListNode p = l1, q = l2, curr = head;
        int carry = 0;
        while (p != null || q != null) {
            int x = (p == null ? 0 : p.val);
            int y = (q == null ? 0 : q.val);
            int sum = x + y + carry;
            carry = sum / 10;
            curr.next = new ListNode(sum % 10);
            curr = curr.next;
            if (p != null)
                p = p.next;
            if (q != null)
                q = q.next;
        }
        if (carry == 1) {
            curr.next = new ListNode(carry);
        }
        return head.next;
    }
}