Leetcode #2 Add two numbers

## 题目

>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 contains 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

---

## 分析

难度中等.

这个题目注意一下是reverse order 意思就是 2 -> 4 -> 3分别是个位,十位,百位

并且l1 & l2的长度不一定是相同的.

解决这道题目的主要是理解两个数相加最大的是进1,不可能进2或者更大的.定义一个carry来记录不进位还是1就好了.

## 解法

```

var addTwoNumbers = function(l1, l2) {

let carry = 0;

let first = new ListNode();

let l3 = first;

while(l1 || l2 || carry) {

// carry 是为了检验如l1 {5} l2{5}这种情况,就是

//  最高位要进位的那种

//  假如不添加,那么 最高位要是进位的话不会被考虑进来.

//  如果不明白可以把while里面的carry去掉 然后测试一下

let sum = carry;

if (l1 !== null) {

sum += l1.val;

l1 = l1.next;

}

if (l2 !== null) {

sum += l2.val;

l2 = l2.next;

}

if (sum > 9) {

carry = 1;

sum = sum-10;

}

else {

carry = 0;

}

l3.next = new ListNode(sum);

l3 = l3.next;

}

return first.next;

};

```