Add Two Numbers

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
/*
 * 1.设置进位为0,并判断列表是否为空
 * 2.两个列表都不为空时,遍历,判断两个列表对应的值相加后是否大于10,大于就设置进位为1,否则为0
 * 3.一个列表为空的时候,进位+列表的对应的值,然后将剩下的添加到结果列表的尾部.
 * 4.两个列表都为空，进位位1，也添加
 */
class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        if (NULL == l1 && NULL == l2) {
            return NULL;
        } else if (NULL == l1) {
            return l2;
        } else if (NULL == l2) {
            return l1;
        }
        ListNode* p1 = l1;
        ListNode* p2 = l2;
        ListNode* res_head = NULL;
        ListNode* res_index = NULL;
        int flag = 0;
        while (p1 != NULL && p2 != NULL) {

            int sum = p1->val + p2->val + flag;
            if (sum >= 10) {
                sum -= 10;
                flag = 1;
            } else {
                flag = 0;
            }

            ListNode* res_node = new ListNode(sum);
            if (NULL == res_head) {
                res_head = res_node;
                res_index = res_head;
            } else {
                res_index->next = res_node;
                res_index = res_node;
            }
            p1 = p1->next;
            p2 = p2->next;
        }

        while (p1 != NULL) {
            int sum = p1->val + flag;
            if (sum >= 10) {
                sum -= 10;
                flag = 1;
            } else {
                flag = 0;
            }

            ListNode* res_node = new ListNode(sum);
            res_index->next = res_node;
            res_index = res_node;
            p1 = p1->next;
        }

        while (p2 != NULL) {
            int sum = p2->val + flag;
            if (sum >= 10) {
                sum -= 10;
                flag = 1;
            } else {
                flag = 0;
            }

            ListNode* res_node = new ListNode(sum);
            res_index->next = res_node;
            res_index = res_node;
            p2 = p2->next;
        }
        
        if (0 != flag) {
            ListNode* res_node = new ListNode(flag);
            res_index->next = res_node;
            res_index = res_node;
        }
        return res_head;
    }
};