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. 主要的单链表操作，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;
    }
    
}

执行结果