2.两数相加(Add Two Numbers) C++

第一想法是顺着题目的原因，将两链表分别转化为一个数字，再将数字相加，然后把结果转化为字符串，存到答案链表中。但是数据太大会溢出！

所以，要在计算一对数字的过程当中直接存储一个结果，注意结果大于9时进位，删去最终链表的最后一个节点。

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        int carry=0;
        int x,y,z;
        ListNode* head = new ListNode(-1);
        ListNode* use = new ListNode(-1);
        ListNode* t = new ListNode(-1);
        head->next = t;
        while(l1 || l2)
        {
            if(!l1)
            {
                x = 0;
            }else x = l1->val;

            if(!l2)
            {
                y = 0;
            }else y = l2->val;

            z = x + y + carry;
            carry = 0;
            if(z>9)
            {
                carry = 1;
                z = z-10;
            }
            t->val = z;
            t->next = new ListNode(-1);
            use = t;
            t = t->next;
            t->next = NULL;

            if(l1)
            {
                l1 = l1->next;
            }
            if(l2)
            {
                l2 = l2->next;
            }
        }
        if(carry)
        {
            t->val = carry;
            t->next = new ListNode(-1);
            use = t;
            t = t->next;
            t->next = NULL;
        }
        t = head->next;
        use->next = NULL;
        return t;
    }
};