【Leetcode】2. Add Two Numbers

Problem：

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.

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

 （分析：相同长度的链表对应val相加，大于10的本位val为0，.next.val加1）

Resolution

public ListNode addTwoNumbers(ListNode l1,ListNode l2){
    int carry =0;//每一位的数值
    ListNode p,result = new ListNode(0);
    p=result;
    while(l1!=null||l2!=null||carry!=0){//注意是l1与l2不为空，而不是l1.next与l2.next不为空
        if(l1!=null){
            carry+=l1.val;
            l1=l1.next;
        }
        if (l2!=null){
           carry+=l2.val;
           l2=l2.next;
        }
        p.next=new ListNode(carry%10);
        carry/10;
        p=p.next;
    }
    return result.next;
}

 注：

①需要保证l1或l2非空。在carry！=0时也要继续进行计算的原因是当l1与l2均进行完加法，到达链表末尾，不会出现溢出问题。

②处理方式并非相对l1与l2的val加和在赋值给carry，而是carry+=l1.val，在carry+=l2.val；

③while循环时，注意是l1与l2不为空，而不是l1.next与l2.next不为空

④ListNode p,result = new ListNode(0); p=result; 初始化的顺序为p，result 后面赋值也必须是这个顺序，否则会报错。

完整可运行代码如下：

package medium;

/**
 * Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
	Output: 7 -> 0 -> 8
 * @author yum717
 *
 */
public class L2 {
	public static class ListNode {
		int val;
		ListNode next;
		ListNode(int x) {
			val = x;
		}
	}

	public  ListNode addTwoNumbers(ListNode l1, ListNode l2) {
	    int carry = 0;
	    ListNode p, result = new ListNode(0);
	    p = result;
	    while (l1 != null || l2 != null || carry != 0) {
	        if (l1 != null) {
	            carry += l1.val;
	            l1 = l1.next;
	        }
	        if (l2 != null) {
	            carry += l2.val;
	            l2 = l2.next;
	        }
	        p.next = new ListNode(carry%10);
	        carry /= 10;
	        p = p.next;
	       
	    } 
　　　　　　　　//输出测试
	    while(result.next!=null){
		System.out.println(result.next.val);
		result=result.next;
	　　}
	    return result.next;
	} 
	public static void main(String[] args) {
		ListNode l1 = new ListNode(2);
		ListNode l2= new ListNode(5);
		ListNode l3 = new ListNode(4);
		ListNode l4= new ListNode(3);
		l1.next=l3;
		l3.next=l4;
		l4.next=null;
		ListNode l23= new ListNode(6);
		ListNode l24 = new ListNode(4);
		l2.next=l23;
		l23.next=l24;
		l4.next=null;
		L2 l= new L2();
    	l.addTwoNumbers(l1,l2);
		
	}
}

 问题：1.实例化一个链表还有什么简洁的的办法？

2.public static class ListNode { int val; ListNode next; ListNode(int x) { val = x; } }

定义链表时，如果不加入static 会出现如下报错信息，不知道如何解释。

Exception in thread "main" java.lang.Error: Unresolved compilation problem: 
	No enclosing instance of type L2 is accessible. Must qualify the allocation with an enclosing instance of type L2
　　　　 (e.g. x.new A() where x is an instance of L2).