addtwonumbers

 * 求两个 List 相加产生的新的一个 List。

 * Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
 * Output: 7 -> 0 -> 8      //注意有进位
 */
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */


public ListNode (ListNode l1, ListNode l2) {
    int carry = 0;                      //初始化进位为0
    ListNode dummy = new ListNode(0);   //头结点
    ListNode cur = dummy;               //由该引用控制添加数据
    while (l1 != null || l2 != null) {
        int a = 0;
        int b = 0;
        if (l1 != null) {
            a = l1.val;
            l1 = l1.next;               //后移一个
        }
        if (l2 != null) {
            b = l2.val;
            l2 = l2.next;
        }
        cur.next = new ListNode((a + b + carry) % 10);
        carry = (a + b + carry) / 10;   //保留进位
        cur = cur.next;                 //后移一个
    }
    if (carry != 0)                     //如果最后的进位不等于0，则另外新添加一个结点
        cur.next = new ListNode(carry);
    return dummy.next;
}