[Leetcode] 2. 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

这道题是CC150上的原题，注意一开始initialize一些变量，那么后面就可以不用重复操作了。time complexity O(n), space complexity O(n)

/** * Definition for singly-linked list. * public class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { *         val = x; *         next = null; *     } * } */public class Solution {    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {        if(l1 == null && l2 == null) return null;        if(l1 == null) return l2;        if(l2 == null) return l1;        ListNode digit = null, previous = null, head = null;        int sum = 0, carry = 0;        while(l1 != null && l2 != null){            sum = l1.val + l2.val + carry;            digit = new ListNode(sum % 10);            carry = sum / 10;            if(head == null){                head = digit;                previous = head;            } else {                previous.next = digit;                previous = previous.next;            }            l1 = l1.next;            l2 = l2.next;        }        ListNode remainder = l1 != null? l1: (l2 != null? l2: null);        while(remainder != null){            sum = remainder.val + carry;            digit = new ListNode(sum % 10);            carry = sum / 10;            previous.next = digit;            previous = previous.next;            remainder = remainder.next;        }        if(carry == 1){            previous.next = new ListNode(1);        }        return head;    }}