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struct ListNode {	int val;	ListNode *next;	ListNode(int x) : val(x), next(NULL) {}};class Solution {//dummy node in the head of the result, avoid judging NULL of headpublic:	ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) {		// Start typing your C/C++ solution below		// DO NOT write int main() function		ListNode dummy(-1);//dummy node in the head of the result, avoid judging NULL of head		ListNode* pre = &dummy;		ListNode* p1 = l1;		ListNode* p2 = l2;		int carry = 0;		while(p1 != NULL || p2 != NULL)		{			int a1 = p1 == NULL ? 0 : p1->val;			int a2 = p2 == NULL ? 0 : p2->val;			int sum = a1+a2+carry;			carry = sum/10;			ListNode* node = new ListNode(sum%10);			pre->next = node;			pre = pre->next;			p1 = p1 == NULL ? NULL : p1->next;			p2 = p2 == NULL ? NULL : p2->next;		}		if(carry != 0) pre->next = new ListNode(carry);		return dummy.next;	}};

second time

/** * 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) {        // Start typing your C/C++ solution below        // DO NOT write int main() function                ListNode dummy(0);        ListNode* prev = &dummy;        int carry = 0;        while(l1 != NULL || l2 != NULL || carry != 0)        {            int a = 0;            int b = 0;                        if(l1 != NULL) a = l1->val, l1 = l1->next;            if(l2 != NULL) b = l2->val, l2 = l2->next;            int sum = a+b+carry;            prev->next = new ListNode(sum%10);            prev = prev->next;            carry = sum/10;        }                return dummy.next;    }};

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