2. 两数相加

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) {
    int index1 = 1, index2 = 1;
    ListNode *temp1 = l1, *temp2 = l2;
    while (temp1->next != nullptr)
        index1++, temp1 = temp1->next;
    while (temp2->next != nullptr)
        index2++, temp2 = temp2->next;

    if (index1 > index2)
        //增加l2
        while (index1 > index2) {
            temp2->next = new ListNode(0);
            temp2 = temp2->next;
            index1--;
        }
    else if (index1 < index2)
        while (index1 < index2) {
            temp1->next = new ListNode(0);
            temp1 = temp1->next;
            index2--;
        }

    //各位相加进位
    temp1 = l1, temp2 = l2;
    auto *obj = new ListNode(-1);
    auto *nextTemp = obj;
    bool flag = false;
    while (temp1 != nullptr) {
        int value = temp1->val + temp2->val;
        if (flag)
            value++, flag = false;
        if (value >= 10)
            flag = true;
        nextTemp->next = new ListNode(value % 10);
        nextTemp = nextTemp->next;
        temp1 = temp1->next;
        temp2 = temp2->next;
    }
    if (flag)
        nextTemp->next = new ListNode(1);
    return obj->next;
}