Leet code submit 주의점 - 전역변수 초기화 | Leet Code Submit Caution - Global Variable Initialization

https://leetcode.com/problems/add-two-numbers/

Add Two Numbers - LeetCode

이문제 풀다가 간단한 문제고, 문제 풀이도 잘 했는데 뭐가 문제지 했었다. 

결론부터 말하자면 LeetCode에서 코드를 제출할 때 static 변수를 사용하지 말자. 사용하더라도 한번의 코드가 끝나면 static 변수를 초기화해주자

public class Solution{
	static int carry = 0;
    
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
    	// carry에 따라 결과값 변화
    }
}

 

대충 요약하면 이런 상황이었는데,

이상하게 testcase에 해당 wrong answer이 나는 걸 그대로 테스트하면 accept 되는데, 실제로 submit을 해서 넣어 돌리면 wrong answer이 떳다. 이상하게 내 output에 이 올바르지 않은 값이었다.

그래서 엥,, 이상하다 하고 leet code 측에 메일을 보냈었는데, 문뜩 생각이 들어서 확인을 해보니, test case를 1000개를 실행해 주었고, 내가 전역변수로 설정한 carry값이 다음 test case 실행에 영향을 미췄더라면? 이라는 가설을 내고 확인해보았더니 맞았다.

리트코드에서 전역변수를 사용할 때는 꼭 전역변수를 초기화해주자. 초기화하지 않은 전역변수 값이 submition시 test case에 영향을 줄 수 있다.

알고리즘을 역시 안풀어보니까 이런거에도 끙끙 대는구나 싶다... 다시 리트코드 풀러가야지 총총

https://leetcode.com/problems/add-two-numbers/

Add Two Numbers - LeetCode

I was solving this problem and it was a simple one — I thought my solution was correct, so I couldn't figure out what was going wrong. 

Long story short: when submitting code on LeetCode, don't use static variables. If you do use them, make sure to reset the static variables after each run.

public class Solution{
	static int carry = 0;
    
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
    	// carry에 따라 결과값 변화
    }
}

 

To summarize, here's what was going on:

Strangely, when I tested the exact wrong answer test case myself, it would pass just fine. But when I actually submitted the code, it came back as a wrong answer. Somehow my output had this incorrect value.

So I was like, huh, that's weird, and I even sent an email to the LeetCode team. But then it suddenly hit me — I checked and found out that they run around 1,000 test cases, and I hypothesized: what if the carry value I declared as a global variable was carrying over and affecting the next test case? I tested that theory, and sure enough, that was exactly the problem.

When using global variables on LeetCode, always make sure to reset them. Uninitialized global variable values can affect other test cases during submission.

I guess this is what happens when you don't practice algorithms regularly — you end up struggling with stuff like this... Time to get back to grinding LeetCode problems. Off I go~