LeetCode: Add two numbers

这题没什么难度，直接看代码吧

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* dfs(ListNode *l1, ListNode *l2, int carry) {
        if (!l1 && !l2) {
            if (!carry) return NULL;
            else {
                ListNode* tmp = new ListNode(carry);
                return tmp;
            }
        }
        if (!l1) {
            int sum = l2->val + carry;
            ListNode* tmp = new ListNode(sum%10);
            carry = sum/10;
            tmp->next = dfs(l1, l2->next, carry);
            return tmp;
        }
        if (!l2) {
            int sum = l1->val + carry;
            ListNode* tmp = new ListNode(sum%10);
            carry = sum/10;
            tmp->next = dfs(l1->next, l2, carry);
            return tmp;
        }
        if (l1 && l2) {
            int sum = l1->val + l2->val + carry;
            ListNode* tmp = new ListNode(sum%10);
            carry = sum/10;
            tmp->next = dfs(l1->next, l2->next, carry);
            return tmp;
        }
    }
    ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int carry = 0;
        ListNode *ret = dfs(l1, l2, carry);
        return ret;
    }
};

 Java：

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        return addTwoNumbersWithCarry(l1, l2, 0);
    }
    private ListNode addTwoNumbersWithCarry(ListNode l1, ListNode l2, int carry)
    {
        if (l1 == null && l2 == null) 
        {
            if (carry > 0)
            {
                return new ListNode(carry);
            }
            else
            {
                return null;
            }
        }
        else if (l1 == null)
        {
            int num = l2.val + carry;
            ListNode newNode = new ListNode(num % 10);
            newNode.next = addTwoNumbersWithCarry(l1, l2.next, num / 10);
            return newNode;
        }
        else if (l2 == null)
        {
            int num = l1.val + carry;
            ListNode newNode = new ListNode(num % 10);
            newNode.next = addTwoNumbersWithCarry(l1.next, l2, num / 10);
            return newNode;
        }
        else
        {
            int num = l1.val + l2.val + carry;
            ListNode newNode = new ListNode(num % 10);
            newNode.next = addTwoNumbersWithCarry(l1.next, l2.next, num / 10);
            return newNode;
        }
    }
}