2.Add Two Numbers

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

代码：

class ListNode{
int val;
ListNode next;
public ListNode(int digit) {
val = digit;
}
}

public static ListNode addTwoNumbers(ListNode l1, ListNode l2) {
if(l1 == null || l2 == null)
return null;

//定义存储变量
int carry = 0;
int sum = l1.val + l2.val;

if(sum > 9){
sum -= 10;
carry++;
}

ListNode head = new ListNode(sum);
ListNode p = head;

ListNode next1 = l1.next;
ListNode next2 = l2.next;

while(next1 != null || next2 != null){
if(next1 == null)
sum = 0 + next2.val + carry;
else if(next2 == null)
sum = next1.val + 0 + carry;
else
sum = next1.val + next2.val + carry;
carry = 0;
if(sum > 9){
sum -= 10;
carry++;
}

ListNode node = new ListNode(sum);
p.next = node;
p = p.next;

if(next1 == null)
next2 = next2.next;
else if(next2 == null)
next1 = next1.next;
else{
next1 = next1.next;
next2 = next2.next;
}
}

if(carry == 1){
ListNode last = new ListNode(1);
p.next = last;
}

return head;
}
}