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Missing test case *(Incorrect/Inefficient Code getting accepted because of missing test cases)* - 1011. Capacity To Ship Packages Within D Days #21823

Closed Sri-Vidya-Guttula closed 4 months ago

Sri-Vidya-Guttula commented 5 months ago

LeetCode Username

Srividya2023

Problem Number, Title, and Link

  1. Capacity To Ship Packages Within D Days https://leetcode.com/problems/capacity-to-ship-packages-within-d-days/description/

Bug Category

Missing test case (Incorrect/Inefficient Code getting accepted because of missing test cases)

Bug Description

Why my test case is important for contributing the test case with:

weights = [3,2,2,4] days = 3

Test case is crucial for improving the quality of"1011. Capacity To Ship Packages Within D Days" on LeetCode's problem validation. Here's why:

Accepted code:

class Solution { public: bool possible_within_days(vector& weights, int days, int mid){ int sum =0; for(int i=0; i<weights.size(); i++){ sum += weights[i]; //cout << sum << " " ; if(days <= 0){ return false; } else if(sum > mid){ sum = weights[i]; days -= 1; if(sum > mid){ return false; } if(sum == mid){ days -= 1; } if(days <= 0){ return false; } //cout << "next" << sum << " "; }

    }
    cout << endl;
    return true;
}
int shipWithinDays(vector<int>& weights, int days) {

    int low = *min_element(weights.begin(), weights.end()), high = 0;
    for(int i=0; i<weights.size(); i++){
        high += weights[i];
    }
    int ans = high ;
    while(low <= high){
        int mid = (low+high)/2;
        if(possible_within_days(weights, days, mid)){

            ans = mid;
            high = mid -1;
        }
        else{
            low = mid + 1;
        }
    }
    return ans;
}

};

The Problem:

The provided code uses binary search to find the minimum capacity required to ship packages within days. It checks if a specific capacity (mid) can ship all packages. However, there's a flaw in the possible_within_days function.

Flaw in the code:

The issue lies in handling the case where the current package weight (weights[i]) exceeds the mid capacity. The code subtracts days by 1 only after it assigns the current package weight to sum. This can lead to an incorrect evaluation:

If sum is already greater than mid before adding the current package, subtracting days by 1 might not be enough. The code might allow exceeding the capacity in some situations. Similarly, if sum becomes equal to mid after adding the current package, subtracting days by 1 might lead to skipping a day unnecessarily in the calculation.

My Test Case Exposes the Flaw:

My test case with weights = [3,2,2,4] and days = 3 perfectly exposes this flaw. Here's what happens:

mid might be initially set to a value like 5 (between minimum and maximum weight). The first three packages (3, 2, and 2) will fit within mid. When the code encounters 4, sum will become 7 which is greater than mid. However, subtracting days by 1 only after assigning 4 to sum might allow the code to proceed with days remaining as 2. This is incorrect because the total weight already exceeds the capacity.

Expected vs. Code Output:

The code might incorrectly report a capacity of 5 (because it "passed" with 2 remaining days). The expected output, however, should be 4. This ensures that no single day's load exceeds the capacity.

Importance of my Contribution:

By including this test case, LeetCode can identify the issue in the code's logic for handling exceeding capacity. This will help them improve the problem validation and ensure submitted solutions accurately solve the problem.

Summary:

My test case plays a vital role in making sure only correct solutions are accepted for this LeetCode problem. It highlights a specific edge case that the current code might miss.

Language Used for Code

C++

Code used for Submit/Run operation

Wrong code:

class Solution {
public:
    bool possible_within_days(vector<int>& weights, int days, int mid){
        int sum =0;
        for(int i=0; i<weights.size(); i++){
            sum += weights[i];
            //cout << sum << " " ;
            if(days <= 0){
                return false;
            }
            else if(sum > mid){
                sum = weights[i];
                days -= 1;
                if(sum > mid){
                    return false;
                }
                if(sum == mid){
                    days -= 1;
                }
                if(days <= 0){
                    return false;
                }
                //cout << "next" << sum  << " "; 
            }

        }
        cout << endl;
        return true;
    }
    int shipWithinDays(vector<int>& weights, int days) {

        int low = *min_element(weights.begin(), weights.end()), high = 0;
        for(int i=0; i<weights.size(); i++){
            high += weights[i];
        }
        int ans = high ;
        while(low <= high){
            int mid = (low+high)/2;
            if(possible_within_days(weights, days, mid)){

                ans = mid;
                high = mid -1;
            }
            else{
                low = mid + 1;
            }
        }
        return ans;
    }
};

correct code:-

class Solution {
public:
    bool possible_within_days(vector<int>& weights, int days, int mid){
        int sum = 0;
        int remaining_days = days; // Track remaining days explicitly
        for(int i=0; i<weights.size(); i++){
            sum += weights[i];
            if(sum > mid){
                remaining_days--; // Decrement remaining days if capacity exceeded
                if(remaining_days <= 0 || weights[i] > mid){ // Check for exceeding capacity in both current and remaining packages
                    return false;
                }
                sum = weights[i]; // Reset sum only if a new day is needed
            }
        }
        return true;
    }
    int shipWithinDays(vector<int>& weights, int days) {

        int low = *min_element(weights.begin(), weights.end()), high = 0;
        for(int i=0; i<weights.size(); i++){
            high += weights[i];
        }
        int ans = high ;
        while(low <= high){
            int mid = (low+high)/2;
            if(possible_within_days(weights, days, mid)){

                ans = mid;
                high = mid -1;
            }
            else{
                low = mid + 1;
            }
        }
        return ans;
    }
};

Expected behavior

Here's an explanation of what was expected to happen in the wrong code vs what actually happened, along with how the correct code addresses it:

Scenario:

LeetCode problem: "1011. Capacity To Ship Packages Within D Days"

Input: weights = [3, 2, 2, 4], days = 3 (This is the test case that exposes the issue)

Wrong Code Behavior:

The possible_within_days function is supposed to check if a specific capacity (mid) can ship all packages within days. When encountering a package weight exceeding mid (4 in this case): It subtracts days by 1 after assigning the current package weight to sum. This can lead to situations where the total weight on a single day goes beyond mid.

Expected Behavior:

The code should ensure no single day's load exceeds the capacity (mid). If the current package weight (4) pushes sum over mid, a new day should be used (decrement days). Only if a new day is needed (days is decremented) should the sum be reset to the current package weight.

How the Correct Code Fixes It:

Introduces remaining_days to explicitly track available days. Checks for exceeding capacity before decrementing remaining_days. This ensures we don't allow exceeding the capacity on a single day. Resets sum to the current package weight only if remaining_days is decremented (indicating a new day is needed).

Explanation for the Test Case:

In the wrong code, with weights = [3, 2, 2, 4] and days = 3: mid might be set to 5 initially. The first three packages (3, 2, and 2) will fit within mid. When encountering 4, sum becomes 7 (exceeding mid). However, subtracting days by 1 only after assigning 4 to sum might allow the code to proceed with days remaining as 2 (incorrect).

The correct code will decrement remaining_days to 2 before checking if 4 exceeds mid on its own. Since 4 itself is greater than mid, it correctly identifies exceeding capacity and returns false. This ensures a minimum capacity of 4 is required to ship the packages within 3 days without exceeding the limit on any single day.

Screenshots

Wrong code accepted by leetcode but failed with my testcase:

Screenshot (175)

correct code accepted by leetocde and succeeded with my testcase:

Screenshot (174)

Additional context

No response

exalate-issue-sync[bot] commented 5 months ago

Joyce_Ndisya commented: Hello,

Your reported issue has been relayed to our team for thorough investigation. We appreciate your patience as we work to address and resolve this matter. We will reach out to you when we have updates regarding the issue.

If you have any further questions or concerns in the meantime, please feel free to let us know.

Best regards, LeetCode Support Team

Sri-Vidya-Guttula commented 5 months ago

May I expect any reply from your end, ma'am?

On Wed, Apr 17, 2024, 5:56 PM exalate-issue-sync[bot] < @.***> wrote:

Joyce_Ndisya commented: Hello,

Your reported issue has been relayed to our team for thorough investigation. We appreciate your patience as we work to address and resolve this matter. We will reach out to you when we have updates regarding the issue.

If you have any further questions or concerns in the meantime, please feel free to let us know.

Best regards, LeetCode Support Team

— Reply to this email directly, view it on GitHub https://github.com/LeetCode-Feedback/LeetCode-Feedback/issues/21823#issuecomment-2061141859, or unsubscribe https://github.com/notifications/unsubscribe-auth/A7JUYHT4GTQ3L5DIG4LWTW3Y5ZS57AVCNFSM6AAAAABGHR7TVKVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDANRRGE2DCOBVHE . You are receiving this because you authored the thread.Message ID: @.***>

exalate-issue-sync[bot] commented 5 months ago

Joyce_Ndisya commented: Hello there,

Thank you for your time.

We appreciate your patience during this time, and we will give you an update as soon as possible.

Please let us know if you have any questions.

Thank you, Joyce

exalate-issue-sync[bot] commented 4 months ago

Joyce_Ndisya commented: Hi there,

Your feedback has been used to enhance the problem. Please provide your LeetCode username to receive your well-deserved 100 LeetCoins reward.

Best regards, LeetCode Team

Sri-Vidya-Guttula commented 4 months ago

Dear LeetCode Team,

Thank you very much for your email and for appreciating my contribution to improving the "1011. Capacity To Ship Packages Within D Days" problem. I'm glad my feedback was helpful!

I'm happy to receive the 100 LeetCoins reward as a token of appreciation. My LeetCode username is splendidsrividya.

Please let me know if there's anything further I can do to assist in improving the quality of LeetCode problems. I'm passionate about algorithmic problem solving and enjoy contributing to the platform's growth.

Thanks again.....

On Mon, Apr 29, 2024, 4:32 PM exalate-issue-sync[bot] < @.***> wrote:

Joyce_Ndisya commented: Hi there,

Your feedback has been used to enhance the problem. Please provide your LeetCode username to receive your well-deserved 100 LeetCoins reward.

Best regards, LeetCode Team

— Reply to this email directly, view it on GitHub https://github.com/LeetCode-Feedback/LeetCode-Feedback/issues/21823#issuecomment-2082429872, or unsubscribe https://github.com/notifications/unsubscribe-auth/A7JUYHXV4U32OGAG64S7MSTY7YSE3AVCNFSM6AAAAABGHR7TVKVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDAOBSGQZDSOBXGI . You are receiving this because you authored the thread.Message ID: @.***>

exalate-issue-sync[bot] commented 4 months ago

Joyce_Ndisya commented: Thank you for your time.

Your feedback has been used to enhance the problem. As a token of our appreciation, your LeetCode account has been credited with 100 LeetCoins.

If you have any more questions or additional feedback, please don't hesitate to let us know. Your continued support is invaluable to us!

Best regards, The LeetCode Team

Sri-Vidya-Guttula commented 4 months ago

That's great, thank you very much.

On Tue, Apr 30, 2024, 11:12 AM exalate-issue-sync[bot] < @.***> wrote:

Joyce_Ndisya commented: Thank you for your time.

Your feedback has been used to enhance the problem. As a token of our appreciation, your LeetCode account has been credited with 100 LeetCoins.

If you have any more questions or additional feedback, please don't hesitate to let us know. Your continued support is invaluable to us!

Best regards, The LeetCode Team

— Reply to this email directly, view it on GitHub https://github.com/LeetCode-Feedback/LeetCode-Feedback/issues/21823#issuecomment-2084423102, or unsubscribe https://github.com/notifications/unsubscribe-auth/A7JUYHSUO4IJH7POLXVDJE3Y74VMZAVCNFSM6AAAAABGHR7TVKVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDAOBUGQZDGMJQGI . You are receiving this because you authored the thread.Message ID: @.***>