题解 | #链表相加(二)#

/**
 * struct ListNode {
 *  int val;
 *  struct ListNode *next;
 *  ListNode(int x) : val(x), next(nullptr) {}
 * };
 */
class Solution {
  public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param head1 ListNode类
     * @param head2 ListNode类
     * @return ListNode类
     */
    ListNode* addInList(ListNode* head1, ListNode* head2) {
        // write code here
        if (head1 == nullptr && head2 == nullptr) {
            return nullptr;
        }
        if (head1 == nullptr) {
            return head2;
        }
        if (head2 == nullptr) {
            return head1;
        }
	  //反转链表
        head1 = ReverseList(head1);
        head2 = ReverseList(head2);
        ListNode* head = nullptr;
        int carry = 0;
        for (; head1 != nullptr &&
                head2 != nullptr; head1 = head1->next, head2 = head2->next) {
            int sum = head1->val + head2->val + carry;
            if (sum / 10 == 1) {
                carry = 1;
            } else {
                carry = 0;
            }
            sum = sum % 10;
            auto q = new ListNode(sum);
            if (head == nullptr) {
                head = q;
                continue;
            }
            q->next = head;
            head = q;
        }
        if (head1 != nullptr &&
                head2 == nullptr) {
            for (; head1 != nullptr; head1 = head1->next) {
                int sum = head1->val + carry;
                if (sum / 10 == 1) {
                    carry = 1;
                } else {
                    carry = 0;
                }
                sum = sum % 10;
                auto q = new ListNode(sum);
                q->next = head;
                head = q;
            }
        } else {
            for (; head2 != nullptr; head2 = head2->next) {
                int sum = head2->val + carry;
                if (sum / 10 == 1) {
                    carry = 1;
                } else {
                    carry = 0;
                }
                sum = sum % 10;
                auto q = new ListNode(sum);
                q->next = head;
                head = q;
            }
        }
        if (carry == 1) {
            auto q = new ListNode(carry);
            q->next = head;
            head = q;
        }
        return head;
    }
    ListNode* ReverseList(ListNode* head) {
        if (head == nullptr) {
            return nullptr;
        }
        if (head->next == nullptr) {
            return head;
        }
        ListNode* p = head->next;
        ListNode* invertHead = head;
        invertHead->next = nullptr;
        while (p->next != nullptr) {
            auto q = p;
            p = p->next;
            q->next = invertHead;
            invertHead = q;
        }
        p->next = invertHead;
        invertHead = p;
        return invertHead;
    }
};