[leetcode] 2. Add Two Numbers

문제

You are given two non-empty linked lists representing two non-negative integers. 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.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

 

풀이

list를 순회하면서 실제 덧셈을 하는 것 같이 계산을 해준다.

받아올림할 변수인 carry를 잘 관리하는 것이 핵심.

 

Time Complexity : $O(n)$

 

코드

/**
 * 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) {
        int carry=0;
        ListNode *ans=new ListNode(0);
        ListNode *p=l1;
        ListNode *q=l2;
        ListNode *now=ans;
        while(p!=NULL || q!=NULL)
        {
            int s =(p?p->val:0)+(q?q->val:0)+carry;
            carry = s/10;
            now->next = new ListNode(s%10);
            now = now->next;
            if(p) p=p->next;
            if(q) q=q->next;
        }
        if(carry) now->next = new ListNode(carry);
        return ans->next;
    }  
};