[算法练习]Add Two Numbers

题目说明：

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

程序代码：

#include <gtest/gtest.h>

using namespace std;

struct ListNode {

    int val;
    ListNode *next;
    ListNode(int x) :
        val(x), next(NULL)
    {}
};

ListNode* addTwoNumbers(ListNode* l1, ListNode* l2)
{
    ListNode* result = 0;
    ListNode* current = 0;
    int carry = 0;

    while (l1 != NULL || l2 != NULL)
    {
        int value = 0;
        if (l1 != NULL)
        {
            value += l1->val;
            l1 = l1->next;
        }

        if (l2 != NULL)
        {
            value += l2->val;
            l2 = l2->next;
        }

        if (current == NULL)
        {
            current = result = new ListNode(value % 10);
            carry = value / 10;
        }
        else
        {
            current->next = new ListNode((value + carry) % 10);
            carry = (value + carry) / 10;
            current = current->next;
        }
    }

    if (carry)
    {
        if (current == NULL)
        {
            current = result = new ListNode(carry);
        }
        else
        {
            current->next = new ListNode(carry);
            current = current->next;
        }
    }

    return result;
}

TEST(Pratices, tAddTwoNumbers)
{
    // (2 -> 4 -> 3) + (5 -> 6 -> 4)  7 -> 0 -> 8
    ListNode* l1 = new ListNode(2);
    l1->next = new ListNode(4);
    l1->next->next = new ListNode(3);

    ListNode* l2 = new ListNode(5);
    l2->next = new ListNode(6);
    l2->next->next = new ListNode(4);

    ListNode* result = addTwoNumbers(l1,l2);

    // NULL + (2 -> 4 -> 3)
    result = addTwoNumbers(NULL,l1);

    // (2 -> 4 -> 3) + (5 -> 6 -> 4 -> 4)
    l2->next->next->next = new ListNode(4);
    result = addTwoNumbers(l1,l2);

    // 1 + 9->9
    l1 = new ListNode(1);
    l2 = new ListNode(9);
    l2->next = new ListNode(9);

    result = addTwoNumbers(l1,l2);
}