2.Add Two Numbers

题目

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

解题思路

1. 基本的单链表操作，注意最后的进位。时间复杂度O(max|m,n|)，空间复杂度O(max|m,n|)。（√）

代码

/** * Definition for singly-linked list. * public class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { val = x; } * } */public class Solution {        public ListNode addTwoNumbers(ListNode l1, ListNode l2) {             ListNode head=null,tail=null;        int first=1;        int sum;        int flag=0;        while(l1!=null || l2!=null){            sum=flag;            if(l1!=null && l2!=null){sum+=l1.val+l2.val;l1=l1.next;l2=l2.next;}            else if(l1!=null){sum+=l1.val;l1=l1.next;}            else {sum+=l2.val;l2=l2.next;}            flag=sum/10;            ListNode node = new ListNode(sum%10);            if(first==1){                first=0;                head=node;                tail=node;            }else{                   tail.next=node;                tail=node;            }                       }                if(flag!=0){            ListNode node = new ListNode(flag);            tail.next=node;        }        return head;    }    }

运行结果