Leetcode Add Two Numbers

熟悉指针的用法。要先虚拟出一个头指针，但是用二级指针应该可以回避这个问题。

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; */class Solution {public:    ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) {        if(!l1 || !l2){            return l1 ? l1 : l2;        }        ListNode *ans = new ListNode(0);        ListNode *cur1 = l1, *cur2 = l2, *head = ans;        int carry = 0;        int sum, n1, n2;        while(cur1 || cur2){            n1 = cur1 ? cur1->val : 0;            n2 = cur2 ? cur2->val : 0;            sum = (n1 + n2 + carry) % 10;            carry = (n1 + n2 + carry) / 10;            ans->next = new ListNode(sum);            cur1 = cur1 ? cur1->next : NULL;            cur2 = cur2 ? cur2->next : NULL;            ans = ans->next;        }        if(carry){            ans->next = new ListNode(1);        }        return head->next;    }};