Leetcode 第二题，两数链表相加

问题：

You are given two linked lists representing two non-negative numbers. 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.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)

Output: 7 -> 0 -> 8

即，两个整数链表，相加，返回一个整数链表，注意进位。

解法：其实这个题名并没有捷径和太多技巧，主要是看是否理解链表的结构。

如果学过C/C++的指针的话，理解起来会更加容易。

/**
* Created by Atlantis on 16/4/25.
*ListNode.java
* Definition for singly-linked list.
*/
public class ListNode {

int val;
ListNode next;

ListNode(int x){
val = x;
}

}

/**
*Solution.java
* Created by Atlantis on 16/4/25.
*/

public class Solution {
public static ListNode addTwoNumbers(ListNode l1, ListNode l2) {
if(null == l1 && null == l2){
return  null;
}

int forward = 0;
ListNode ret = new ListNode(0);
ListNode head = ret;
while (null != l1 && null != l2){
int sum = l1.val + l2.val +forward;
forward = sum/10;
l1 = l1.next;
l2 = l2.next;
ret.next = new ListNode(sum%10);
ret = ret.next;
}
while (null != l1){
int sum = l1.val+forward;
ret.next = new ListNode(sum%10);
forward = sum/10;
ret = ret.next;
l1 = l1.next;
}
while (null != l2){
int sum = l2.val+forward;
ret.next = new ListNode(sum%10);
forward = sum/10;
ret = ret.next;
l2 = l2.next;
}
if(forward != 0 ){
ret.next = new ListNode(forward);
}

return head.next;
}

public static void main(String[] args){
ListNode l1 = new ListNode(6);
l1.next = new ListNode(4);
l1.next.next = new ListNode(3);

ListNode l2 = new ListNode(5);
l2.next = new ListNode(6);
l2.next.next = new ListNode(4);

ListNode ret =addTwoNumbers(l1,l2);
if(null != ret) {
System.out.print(ret.val);
}
while (ret.next != null){
System.out.print("->");
System.out.print(ret.next.val);
ret = ret.next;

}
}
}

结束：难度不是很大，会一点C/C++，比起双向链表什么的，有点无聊。。。

链表这个东西，果然还是指针理解起来比较容易和直观。