题解 | #两个链表生成相加链表#

链表翻转后节点相加(使用栈模拟)，如果有进位则记录，产生新的结点以头插法插入到新的链表;注意加完以后需要检查最后是否有进位，如果有进位就还需要再插入一个新的节点。

/**
 * 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* addInList(ListNode* head1, ListNode* head2) {
        // write code here
        //链表翻转后节点相加(使用栈模拟)，如果有进位则记录，产生新的结点以头插法插入到新的链表;
        ListNode* rst = NULL;

        stack<ListNode*> head1_stack;
        stack<ListNode*> head2_stack;

        ListNode* p = head1;
        ListNode* q = head2;

        while(p)
        {
            head1_stack.push(p);
            p=p->next;
        }
        while(q)
        {
            head2_stack.push(q);
            q=q->next;
        }

        int carry = 0;
        while(!head1_stack.empty() && !head2_stack.empty())
        {
            p = head1_stack.top();
            q = head2_stack.top();
            int sum = p->val + q->val + carry;
            carry = sum / 10;
            sum = sum % 10;
            rst = insert_node(rst,sum);
            head1_stack.pop();
            head2_stack.pop();
        }

        if(!head1_stack.empty())
        {
            //head1更长;
            while(!head1_stack.empty())
            {
                p = head1_stack.top();
                int sum = p->val + carry;
                carry = sum / 10;
                sum = sum % 10;
                rst = insert_node(rst,sum);
                head1_stack.pop();
            }
        }else if(!head2_stack.empty())
        {
            //head2更长;
            while(!head2_stack.empty())
            {
                q = head2_stack.top();
                int sum = q->val + carry;
                carry = sum / 10;
                sum = sum % 10;
                rst = insert_node(rst,sum);
                head2_stack.pop();
            }
        }

        //看最后是否有进位;
        if(carry > 0)
        {
            rst = insert_node(rst,carry);
        }

        return rst;
    }

    //头插法插入节点，返回头指针;
    ListNode* insert_node(ListNode* head, int n)
    {
        ListNode* new_node = new ListNode(n);
        if(head == NULL)
        {
            head = new_node;
        }else{
            new_node->next = head;
            head = new_node;
        }
        return head;
    }
};