2. 两数相加

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */

// LeetCode指定调用方法 
public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
    ListNode head = null;
    ListNode tail = null;
    int carry = 0;
    while (l1 != null || l2 != null) {
        int num1 = l1 != null ? l1.val : 0;
        int num2 = l2 != null ? l2.val : 0;
        int sum = num1 + num2 + carry;
        int num = sum % 10;
        carry = sum / 10;
        if (head == null) {
            head = new ListNode(num);
            tail = head;
        } else {
            tail.next = new ListNode(num);
            tail = tail.next;
        }

        if (l1 != null) {
            l1 = l1.next;
        }
        if (l2 != null) {
            l2 = l2.next;
        }
    }
    if (carry != 0) {
        tail.next = new ListNode(carry);
    }
    return head;
}