Leetcode(2) – Add Two Numbers

https://leetcode.com/problems/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

 

这道题思路比较简单，两个链表从头到尾相加即可。容易出错的地方是最后两个链表走完要记得check offset是否为0. 不为0还要加上一个node. 此外这种从头构造链表的题目，技巧是加一个dummy node。代码如下，时间复杂度O(n), 空间复杂度O(1)

/**
 * 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 dummy = new ListNode(0);
        ListNode tail = dummy;
        int offset = 0;
        while(l1 != null || l2 != null){
            int total = 0;
            if(l1 != null){
                total += l1.val;
                l1 = l1.next;
            }
            if(l2 != null){
                total += l2.val;
                l2 = l2.next;
            }
            total += offset;
            ListNode node = new ListNode(total%10);
            offset = total / 10;
            tail.next = node;
            tail = node;
        }
        if(offset != 0){
            ListNode node = new ListNode(offset);
            tail.next = node;
        }
        return dummy.next;
    }
}