• 问题
  You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

• Example
  Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
  Output: 7 -> 0 -> 8

• 解决

package com.lsy.leetcode;

public class AddTwoNumbers {

     static AddTwoNumbers a=new AddTwoNumbers();
     public static ListNode addTwoNumbers(ListNode l1, ListNode l2) {
//       if (l1 == null) {       //说了非空
//              return l2;
//          }
//
//          if (l2 == null) {
//              return l1;
//          }
            int carry = 0;
            
            ListNode newhead = a.new ListNode(-1);
            ListNode l3 = newhead;
            //ListNode l3;
            while(l1!=null || l2!=null){
                  if(l1!=null){
                      carry += l1.val;
                     // System.out.println(l1.val+"test");
                      l1 = l1.next;
                  }
                  if(l2!=null){
                      carry += l2.val;
                      l2 = l2.next;
                  }
                  //l3.next = a.new ListNode(carry%10);  //测试不能从l3开始也可以的
                  l3.val=(carry%10);//那就不是指向newhead了，需要加一个改变val的方法
                  System.out.println(newhead.val);
                  carry = carry/10;
                  l3.next=a.new ListNode(-1);//用之前实例化l3;
                  l3=l3.next;   //不能和上一行调换位置
            }
            
            if(carry == 1)
                 l3.next=a.new ListNode(1);
          
           // return newhead.next;
           // System.out.println(newhead.val+"->"+newhead.next.val+"->"+newhead.next.next.val);
            return newhead; 
        }
            
    
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        
        ListNode l1=a.new ListNode(2);  //内部类，不懂呢
        l1.next=a.new ListNode(4);
        l1.next.next=a.new ListNode(3);
        
        ListNode l2=a.new ListNode(5);  //内部类，不懂呢
        l2.next=a.new ListNode(6);
        l2.next.next=a.new ListNode(4);
        //l1.next=new ListNode(4);
        
        ListNode l3=addTwoNumbers(l1,l2);
        System.out.println(l3.val+"->"+l3.next.val+"->"+l3.next.next.val);
        System.out.println(l1.val);  //测试引用传递，l1没有变。传递的是引用的复制。
                                     //参考：http://www.cnblogs.com/lixiaolun/p/4311863.html
    }

     class ListNode {
              int val;
              ListNode next;
              public ListNode(int x) { val = x; }
          }
     
     public ListNode addTwoNumbers2(ListNode l1, ListNode l2) {
            ListNode dummyHead = new ListNode(0);
            ListNode p = l1, q = l2, curr = dummyHead;
            int carry = 0;
            while (p != null || q != null) {
                int x = (p != null) ? p.val : 0;   //先确定下来
                int y = (q != null) ? q.val : 0;
                int sum = carry + x + y;  
                carry = sum / 10;              //取整
                curr.next = new ListNode(sum % 10);  //取余
                curr = curr.next;
                if (p != null) p = p.next;
                if (q != null) q = q.next;
            }
            if (carry > 0) {
                curr.next = new ListNode(carry);
            }
            return dummyHead.next;
        }
}