LeetCode【2】-Add Two Numbers

题目

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

给定两个非空的链表，表示两个非负整数。 数字以相反的顺序存储，每个节点包含一个数字。 添加两个数字并将其作为链表返回。
您可以假设两个数字不包含任何前导零，除了数字0本身。

也就是从头加到尾，需要注意的是如何处理不同长度的数字，以及进位和最高位的判断。这里对于不同长度的数字，我们通过将较短的数字补0来保证每一位都能相加。递归写法的思路比较直接，即判断该轮递归中两个ListNode是否为null。

• 全部为null时，返回进位值
• 有一个为null时，返回不为null的那个ListNode和进位相加的值
• 都不为null时，返回 两个ListNode和进位相加的值

代码

public class Solution {public ListNode addTwoNumbers(ListNode l1, ListNode l2) {    return helper(l1,l2,0);}public ListNode helper(ListNode l1, ListNode l2, int carry){    if(l1==null && l2==null){        return carry == 0? null : new ListNode(carry);    }    if(l1==null && l2!=null){        l1 = new ListNode(0);    }    if(l2==null && l1!=null){        l2 = new ListNode(0);    }    int sum = l1.val + l2.val + carry;    ListNode curr = new ListNode(sum % 10);    curr.next = helper(l1.next, l2.next, sum / 10);    return curr;    }}

完整运行程序：

package AddTwoNum;class ListNode{    int val;    ListNode nextNode;    ListNode(int val){        this.val = val;        this.nextNode = null;    }}public class AddTwoNum {     public ListNode addTwoNumbers1(ListNode l1, ListNode l2) {            return helper(l1,l2,0);        }     public ListNode helper(ListNode l1,ListNode l2,int carry){         if(l1 == null && l2 == null){             return carry == 0? null : new ListNode(carry);         }         if(l1 == null && l2 != null){             l1 = new ListNode(0);         }         if(l2 == null && l1 != null){             l2 = new ListNode(0);         }         int sum = l1.val + l2.val+carry;         ListNode curr = new ListNode(sum%10);         curr.nextNode = helper(l1.nextNode, l2.nextNode, sum/10);         return curr;     }     public static void main(String[] args) {        int []input1 = new int []{2,4,3};        int []input2 = new int []{5,6,4};        ListNode listNode1 = buildListNode(input1);        ListNode listNode2 = buildListNode(input2);        AddTwoNum addTwoNum = new AddTwoNum();        ListNode result = addTwoNum.addTwoNumbers1(listNode1,listNode2);        while(result != null){            System.out.print(result.val);            result = result.nextNode;            if(result !=null){                System.out.print("->");            }        }    }     private static ListNode buildListNode(int[] input){         ListNode first = null,last = null,newNode;         int sum;         if (input.length>0){             for(int i = 0;i<input.length;i++){                 newNode = new ListNode(input[i]);                 newNode.nextNode = null;                 if(first == null){                     first = newNode;                     last = newNode;                 }else {                    last.nextNode= newNode;                    last = newNode;                }             }         }         return first;     }}