[leetCode]Add Two Numbers

想当初写过C的大数加减法，恍如隔世。

#include <iostream>
using namespace std;
struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};
class Solution {
public:
    ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) {
        if(l1 == NULL) return l2;
        if(l2 == NULL) return l1;
        ListNode *head,*tail;
        head = tail = NULL;
        bool isCarry = false;
        while(l1 != NULL && l2 != NULL){
            int digit = l1->val + l2->val;
            if(isCarry) digit++;
            if(digit >= 10){
                digit %= 10;
                isCarry = true;
            }else{
                isCarry = false;
            }
            if(head == NULL) head = tail = new ListNode(digit);
            else{
                tail->next = new ListNode(digit);
                tail = tail->next;
            }
            l1 = l1->next;
            l2 = l2->next;
        }
        ListNode *cur = NULL;
        if(l1 == NULL && l2 == NULL){
            if(isCarry) tail->next = new ListNode(1);
            return head;
        }
        else if(l1 == NULL) cur = l2;
        else if(l2 == NULL) cur = l1;
        tail->next = cur;
        while(isCarry){
            cur->val +=1;
            if(cur->val < 10) isCarry = false;
            else{
                cur->val %= 10;
                if(cur->next == NULL) cur->next = new ListNode(0);
                cur = cur->next;
            }
        }
        return head;
    }
};