题解 | #两个链表生成相加链表#

记下来，坑很多不能大意，首先就是模拟进位加法而不是直接转化为int！，然后注意要翻转列表，因为进位加法从低位开始运算，然后是要熟悉进位加法，尤其是进位不要忘记了！

#include <stack>
/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */

class Solution {
public:
    /**
     * 
     * @param head1 ListNode类 
     * @param head2 ListNode类 
     * @return ListNode类
     */
    ListNode* reverse(ListNode* head){
        ListNode *next = head->next;
        head->next = NULL;
        while(next != NULL){
            ListNode* nextnext = next->next;
            next->next = head;
            head = next;
            next = nextnext;
            
        }
        return head;
    }
    
    ListNode* addInList(ListNode* head1, ListNode* head2) {
        // write code here
        if(head1 == NULL)
            return head2;
        if(head2 == NULL)
            return head1;
        head1 = reverse(head1);
        head2 = reverse(head2);
        ListNode* head = NULL;
        ListNode* cur = NULL;
        int jin = 0;
        while(head1 != NULL and head2 != NULL){
            int val1 = head1->val;
            int val2 = head2->val;
            int sum = val1 + val2 + jin;
            jin = sum / 10;
            int remain = sum % 10;
            ListNode* next = new ListNode(remain);
            if(head == NULL){
                head = next;
                cur = next;
            }
            else{
                cur->next = next;
                cur = next;
            }
            head1 = head1->next;
            head2 = head2->next;
        }
        while(head1 != NULL){
            int sum = head1->val + jin;
            int remain = sum % 10;
            jin = sum / 10;
            ListNode* next = new ListNode(remain);
            cur->next = next;
            cur = next;
            head1 = head1->next;
        }
        while(head2 != NULL){
            int sum = head2->val + jin;
            int remain = sum % 10;
            jin = sum / 10;
            ListNode* next = new ListNode(remain);
            cur->next = next;
            cur = next;
            head2 = head2->next;
        }
        if(jin != 0){
            ListNode* next = new ListNode(jin);
            cur->next = next;
            cur = next;
        }
        head = reverse(head);
        return head;
            
    }
};