个人记录-LeetCode 2.Add Two Numbers

问题：

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

代码示例：

/** * 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) {
        int length1 = 0;
        ListNode temp = l1;
        while (temp != null) {
            length1++;
            temp = temp.next;
        }

        int length2 = 0;
        temp = l2;
        while(temp != null) {
            length2++;
            temp = temp.next;
        }

        int longLength = length1 > length2 ? length1 : length2;
        if (longLength == 0) {
            return null;
        }

        ListNode result = new ListNode(0);
        ListNode realResult = result;

        int carry = 0;

        for (int i = 0; i < longLength; ++i) {
            int first = 0;
            int second = 0;

            if (l1 != null) {
                first = l1.val;
                l1 = l1.next;
            }

            if (l2 != null) {
                second = l2.val;
                l2 = l2.next;
            }

            result.val = carry + first + second;
            if (result.val >= 10) {
                result.val = result.val - 10;
                carry = 1;
            } else {
                carry = 0;
            }

            //we should not make a listNode every time
            if (i < longLength -1) {
                result.next = new ListNode(0);
                result = result.next;
            }
        }

        //only if we have a carry, then we create a new listnode for the last addition
        if (carry == 1) {
            result.next = new ListNode(0);
            result.next.val = carry;
        }

        return realResult;
    }
}