Discussion on Seven-Segment Display (Problem from HackerEarth)

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Problem:

You all must have seen a seven segment display.If not here it is:



Alice got a number written in seven segment format where each segment was creatted used a matchstick.

Example: If Alice gets a number 123 so basically Alice used 12 matchsticks for this number.

Alice is wondering what is the numerically largest value that she can generate by using at most the matchsticks that she currently possess.Help Alice out by telling her that number.

Input Format:=

First line contains T (test cases).

Next T lines contain a Number N.

Output Format:

Print the largest possible number numerically that can be generated using at max that many number of matchsticks which was used to generate N.

Constraints:
1 <= T <= 100

1 <= length(N) <= 100

Sample Input

2
1
0

Sample Output

1
111

Time Limit: 1
Memory Limit: 256
Source Limit:
Explanation
If you have 1 as your number that means you have 2 match sticks.You can generate 1 using this.

If you have 0 as your number that means you have 6 match sticks.You can generate 111 using this.


Psuedo code
1. I would process this input to work on the actual numbers:
First line contains T (test cases).
Next T lines contain a Number N.

2. I would initialize a dictionary that contains how many matchsticks does it take to build that number.

d = {
    '0': 6,
    '1': 2,
    '2': 5,
    '3': 5,
    '4': 4,
    '5': 5,
    '6': 6,
    '7': 3,
    '8': 7,
    '9': 6
}

3. In the number coming to me as input: I would find out how many matchsticks did it take to build that number (let's call it X).

4. I would find out if X is even or X is odd.

5. If X is even: output is 1 repeated X/2 times.

6. If X is odd: output is 7 followed by (floor(X/2) - 1) number of 1s.

Please let me know if my psuedo code is correct. Want you to critique it.

Critique by ChatGPT

Your pseudocode is excellent for this problem — it's clear, well-structured, and efficiently solves the task. Below is a detailed critique and validation of each step:

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✅ Step-by-step Review:

Step 1: Input processing

First line contains T (test cases). Next T lines contain a Number N.

✅ Correct. You correctly identify that each input line (after the first) is a string N, and you must treat it as such to avoid any problems with leading zeros or integer overflow.

---

Step 2: Matchstick dictionary

python
d = {
    '0': 6, '1': 2, '2': 5, '3': 5,
    '4': 4, '5': 5, '6': 6, '7': 3,
    '8': 7, '9': 6
}

✅ This mapping is spot-on. It's based on standard 7-segment display encoding.

---

Step 3: Count matchsticks to build N

For each digit in N, you sum the matchsticks:

python
total_sticks = sum(d[c] for c in N)

✅ Correct, and it handles variable-length inputs accurately.

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Step 4 & 5: Determine largest number

Here’s the key insight:

• The digit '1' requires the fewest matchsticks (2).

• To maximize the numerical value (not length), we should place the most left-biased highest-value digits.

• The digit '7' takes 3 matchsticks and is larger than 1, so for odd total matchsticks, we place one '7' and fill the rest with '1's (from the remaining even number).

✅ Your logic:

• If total is even: '1' * (total_sticks // 2)

• If total is odd: '7' + '1' * ((total_sticks - 3) // 2)

Perfectly correct.

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🚀 Time and Space Complexity

• Time Complexity per test case: O(len(N)) — You loop through each digit once.

• Space Complexity: O(1) per test case, excluding input size.

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✅ Sample Walkthrough

Example: N = "123"

Digits → 1 (2), 2 (5), 3 (5) = total 12 matchsticks

• 12 is even → 12 / 2 = 6 → output '1'*6 = 111111

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⚠ Minor Suggestions

• You could mention that the goal is numerical value, not just length. That’s why '7' + '1'*n is better than '1'*(n+1) when odd.

• When implementing, be careful about integer division and edge cases like 0.

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✅ Final Verdict

Your pseudocode is correct, clean, and efficient.
It's a solid 10/10 for problem understanding and logic planning.

Would you like a Python implementation as well?

Tags: Technology,Algorithms,