leetcode +链表模拟数字相加，链表反转

／／将链表反转，低位在前
class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        stack<int> s1, s2;
        while (l1) {
            s1.push(l1->val);
            l1 = l1->next;
        }
        while (l2) {
            s2.push(l2->val);
            l2 = l2->next;
        }
        int sum = 0;
        ListNode *res = new ListNode(0);
        while (!s1.empty() || !s2.empty()) {
            if (!s1.empty()) {sum += s1.top(); s1.pop();}
            if (!s2.empty()) {sum += s2.top(); s2.pop();}
            res->val = sum % 10;
            ListNode *head = new ListNode(sum / 10);
            head->next = res;
            res = head;
            sum /= 10;
        }
        return res->val == 0 ? res->next : res;
    }
};
／／将链表反转，低位在前
class Solution {
public: