2. Add Two Numbers

You are given two linked lists representing two non-negative numbers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4) Output: 7 -> 0 -> 8

/**
 * 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) {
        ListNode fakeHead = ListNode(0);
        ListNode *p = &fakeHead;

        int carry = 0, sum = 0;

        while(l1 || l2){
            sum = l1->val + l2->val + carry;
            carry = sum/10;
            sum = sum % 10;
            p->next = new ListNode(sum);
            p = p->next;
            l1 = l1->next;
            l2 = l2->next;

            if(l2 == NULL){
                while(l1 != NULL){
                    sum = l1->val + carry;
                    carry = sum/10;
                    sum = sum % 10;
                    p->next = new ListNode(sum);
                    p = p->next;
                    l1 = l1->next;
                }
            }

            if(l1 == NULL){
                while(l2 != NULL){
                    sum = l2->val + carry;
                    carry = sum/10;
                    sum = sum % 10;
                    p->next = new ListNode(sum);
                    p = p->next;
                    l2 = l2->next;
                }
            }
        }
        if(carry)
            p->next = new ListNode(carry);
        return fakeHead.next;
    }
};

精简版本

/**
 * 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) {
        ListNode fakeHead = ListNode(0);
        ListNode *p = &fakeHead;

        int carry = 0;

        while(l1 || l2){
            int sum = 0;
            if(l1 != NULL){
                sum += l1->val;
                l1 = l1->next;
            }
            if(l2 != NULL){
                sum += l2->val;
                l2 = l2->next;
            }
            sum += carry;
            p->next = new ListNode(sum % 10);
            p = p->next;
            carry = sum /10;
        }
        if(carry)
            p->next = new ListNode(carry);
        return fakeHead.next;
    }
};