[数据结构与算法]链表求和。

分析：
定义三个链表，两个链表负责两组数据的存储，第三个链表负责存储结果，前两个链表每个对应位置上的数据相加，注意进位处理，以及如果两个链表不等长的情况。

代码实现：

class Solution3 {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        if(l1 == null)
            return l2;
        if(l2 == null)
            return l1;
        ListNode head = new ListNode(0);
        ListNode cur = head;
        int num = 0;
        int flag = 0;
        while(l1!=null || l2!=null || flag!=0){
            int num1 = 0;
            int num2 = 0;
            if(l1 != null){
                num1 = l1.data;
                l1 = l1.next;
            }
            if(l2 != null){
                num2 = l2.data;
                l2 = l2.next;
            }
            num = (num1 + num2 + flag) % 10;
            flag = (num1 + num2 + flag) / 10;
            ListNode t = new ListNode(num);
            cur.next = t;
            cur = cur.next;
        }
        return head.next;
    }
}
public class ListNodeAdd{
    public static void main(String[] args){
        Solution3 s = new Solution3();
        ListNode l1 = null;
        ListNode l11 = new ListNode(7);
        ListNode l12 = new ListNode(1);
        ListNode l13 = new ListNode(6);
        l1 = l11;
        l11.next = l12;
        l12.next = l13;

        ListNode l2 = null;
        ListNode l21 = new ListNode(5);
        ListNode l22 = new ListNode(9);
        ListNode l23 = new ListNode(2);
        l2 = l21;
        l21.next = l22;
        l22.next = l23;
        System.out.println(l2);
        ListNode lnew = s.addTwoNumbers(l1,l2);
    }
}

public class ListNode {
    public int data;
    public ListNode next;
    public ListNode(int data){
        this.data = data;
    }
}