What is the time complexity of Maximum 69 Number?

The optimal solution has a time complexity of O(n).

What data structures are used to solve Maximum 69 Number?

This problem uses Math, Greedy. Understanding these concepts is essential for solving this Easy problem efficiently.

What are the key insights for solving Maximum 69 Number?

**Maximization through Leftmost Impact:** To achieve the maximum number, changes should prioritize higher place values. Converting the leftmost '6' to a '9' ensures the largest possible increase, as it modifies the most significant digit that can be improved. **Unidirectional Change Focus:** The problem's objective to 'maximize' the number means we only care about transforming '6's into '9's. Flipping a '9' to a '6' would always decrease the number, thus it's an operation we actively avoid.

Which companies ask LeetCode 1323. Maximum 69 Number in interviews?

This problem is frequently asked at Hudson River Trading.

JavaScript is required for the full interactive experience. However, all content is accessible without JavaScript.

Maximum 69 Number - LeetCode 1323 Solution

Maximum 69 Number - Complete Solution Guide

Maximum 69 Number is LeetCode problem 1323, a Easy level challenge. This complete guide provides step-by-step explanations, multiple solution approaches, and optimized code in python3, java, cpp, c.

Problem Statement

You are given a positive integer num consisting only of digits 6 and 9 . Return the maximum number you can get by changing at most one digit ( 6 becomes 9 , and 9 becomes 6 ) . Example 1: Input: num = 9669 Output: 9969 Explanation: Changing the first digit results in 6669. Changing the second digit results in 9969. Changing the third digit results in 9699. Changing the fourth digit results in 9666. The maximum number is 9969. Example 2: Input: num = 9996 Output: 9999 Explanation: Changing the la

Detailed Explanation

Imagine you're given a number like `9669`, but with a twist: it's guaranteed to be made up *only* of the digits `6` and `9`. Your task is to find the largest possible number you can construct by flipping *at most one* digit. A `6` becomes a `9`, and a `9` becomes a `6`. The 'at most one' part is crucial – you can choose to make no change if that's the best option, or change just one digit.

Solution Approach

The core idea here is a greedy one, driven by the principles of place value. To make an integer as large as possible, you want its leftmost (most significant) digits to be the biggest they can be. Given that our only options are 6s and 9s, and we can only change *one* digit, the most impactful change we can make to increase the number is to turn a `6` into a `9`. Crucially, this change should happen at the highest possible place value – meaning, the leftmost `6` you encounter. Changing a `9` to a `6` would always decrease the number, which is counterproductive to our goal of maximization. Therefore, the strategy simplifies to: find the very first (leftmost) `6` in the number and transform it into a `9`. If there are no `6`s, the number is already all `9`s, which is the maximum possible state, so no change is needed or beneficial. The provided solution elegantly implements this by converting the integer to a string, using Python's `str.replace('6', '9', 1)` to replace only the *first* occurrence of '6' with '9', and then converting it back to an integer.

Step-by-Step Algorithm

Step 1: Convert the input integer `num` to a string `s`.
Step 2: Iterate through the string `s` from left to right.
Step 3: If a digit '6' is encountered, replace it with '9' and immediately break the loop.
Step 4: Convert the modified string `s` back to an integer and return it. If no '6' was found, return the original `num`.

Key Insights

**Maximization through Leftmost Impact:** To achieve the maximum number, changes should prioritize higher place values. Converting the leftmost '6' to a '9' ensures the largest possible increase, as it modifies the most significant digit that can be improved.
**Unidirectional Change Focus:** The problem's objective to 'maximize' the number means we only care about transforming '6's into '9's. Flipping a '9' to a '6' would always decrease the number, thus it's an operation we actively avoid.
**String Conversion for Direct Manipulation:** Representing the number as a string simplifies the task of identifying and modifying a specific digit based on its position. Python's `str.replace()` method, with its `count` argument, directly handles the 'at most one digit' constraint and targets the 'first' relevant digit efficiently.

Complexity Analysis

Time Complexity: O(n)

Space Complexity: O(n)

Topics

This problem involves: Math, Greedy.

Companies

Asked at: Hudson River Trading.

Maximum 69 Number | LeetCode 1323

Easy

Math Greedy

Time: O(n)Space: O(n)

Solution Code

Available in:

class Solution {
    public int maximum69Number (int num) {
        String s = String.valueOf(num);
        for(int i=0; i<s.length(); i++){
            if(s.charAt(i) == '6'){
                return Integer.parseInt(s.substring(0,i) + '9' + s.substring(i+1));
            }
        }
        return num;
    }
}

View Full Solution on GitHubGitHub Solution View on LeetCodeLeetCode

Problem Info

Description

Problem 1323: Maximum 69 Number - Detailed explanation and solution approaches.

Topics

Math Greedy

Companies

Hudson River Trading

Stats

Difficulty:Easy

Languages:4

Companies:1

Problem #:1323

Problem Explanation

💡

Think Before You Code

🤔 Ask Yourself

→What is the input? What output do I need?
→Can I solve a smaller version of this problem first?
→What patterns have I seen in similar problems?

✏️ Try This First

Before looking at the solution, try to solve it on paper with a simple example!

🚀 Try on LeetCode

Solution Approach

📋

Step-by-Step Solution

4 steps

1
Step 1: Convert the input integer `num` to a string `s`.
2
Step 2: Iterate through the string `s` from left to right.
3
Step 3: If a digit '6' is encountered, replace it with '9' and immediately break the loop.
4
Step 4: Convert the modified string `s` back to an integer and return it. If no '6' was found, return the original `num`.

💡

Pro Tip for Students

Try tracing through each step with a small example on paper before coding. Understanding "why" each step happens is more important than memorizing the code!

⚡

Complexity Analysis

⏱️

Time Complexity

O(n)

👍 Linear - Good! Visits each element once.

💾

Space Complexity

O(n)

📦 Linear space - Memory grows with input size.

📝

Why?

The dominant operation is the single iteration over the string representation of the number. String conversion and the back-conversion are also linear in the number of digits. Space is used to store the string representation; hence the linear space complexity.

📖New to Big O? Click to learn more

Big O Notation - Quick Guide

O(1)Best - Instant

O(log n)Excellent - Binary Search

O(n)Good - One loop

O(n log n)OK - Sorting

O(n²)Slow - Nested loops

O(2ⁿ)Very Slow - Recursion

Alternative Approaches

Mathematical Approach

Using mathematical operations to find the place value of the first 6 and adjusting the number accordingly.

Time: O(log n) to determine the place valueSpace: O(1)

Trade-offs: While slightly more efficient in terms of time complexity for very large numbers, the string manipulation approach tends to be easier to understand and implement.

✨

Key Insights

1**Maximization through Leftmost Impact:** To achieve the maximum number, changes should prioritize higher place values. Converting the leftmost '6' to a '9' ensures the largest possible increase, as it modifies the most significant digit that can be improved.
2**Unidirectional Change Focus:** The problem's objective to 'maximize' the number means we only care about transforming '6's into '9's. Flipping a '9' to a '6' would always decrease the number, thus it's an operation we actively avoid.
3**String Conversion for Direct Manipulation:** Representing the number as a string simplifies the task of identifying and modifying a specific digit based on its position. Python's `str.replace()` method, with its `count` argument, directly handles the 'at most one digit' constraint and targets the 'first' relevant digit efficiently.

📚

Key Terms Explained

For Students

Greedy Algorithm

Making the best choice at each step without looking ahead.

Edge Cases

Edge case 1: The input integer contains only the digit '9'. The solution correctly handles this by returning the original number since no improvement is possible.
Edge case 2: The input integer is a single digit '6'. The solution correctly changes this to '9'.

⚠️

Common Mistakes to Avoid

✗Common mistake 1: Incorrectly handling the case where the input contains only 9s. Forgetting to return the original number in this scenario leads to an incorrect output.

✗Common mistake 2: Inefficiently searching for the '6'. Using a nested loop or redundant string manipulations makes the solution less efficient.

✅

Learning Tip

Every mistake is a learning opportunity! When you make an error, understand why it happened before moving on.

🌍

Where is This Used in Real Life?

💼Real-world scenario 1: Optimizing a numerical code where a small change can significantly improve a value (e.g., a version number).

🏦Real-world scenario 2: Simple data transformation tasks, where a specific pattern needs to be altered in a string representation of numbers.

💡Understanding real-world applications helps you see the "why" behind algorithms. It's not just about passing interviews—these concepts power the apps and websites you use every day!

Author's Notes

💡 Key Insight

When dealing with arrays, always consider if sorting would simplify the problem. Many array problems have O(n) solutions using hash maps or two-pointer technique.

🎯 Interview Tip

Even for easy problems, discuss time and space complexity. Interviewers want to see you think analytically.

⚠️ Watch Out

Always test your solution with edge cases: empty input, single element, and boundary values.

Written by Saswata Mondal

Software Engineer • Algorithms Enthusiast • Interview Coach

👨‍💻

About the Author

👨‍💻

Saswata MondalAuthor400+ LC Problems

B.Tech CSE, IEM Kolkata | LeetCode Rating: 1672 (Top 15%)

Certified: IBM Java Developer, NPTEL Programming in Java

Last updated: August 3, 2024LeetCode Profile GitHub

This solution has been crafted based on my experience solving 400+ LeetCode problems. Each explanation is designed to help you understand the underlying concepts, not just memorize the code.

Community Tips & Insights

Try solving this without using extra space first. It's a great exercise for understanding the underlying pattern.

@CodeNinjaJul 14

In a real interview, always clarify constraints first. Ask about input size and value ranges.

@TechLeadJul 7

Don't forget to handle the edge case when the input is empty. Many candidates miss this in interviews.

@DevExpert92Jun 30

❓

Frequently Asked Questions

What are common greedy problem patterns?

Activity selection (sort by end time), interval scheduling (sort by start/end), Huffman coding (always combine smallest), fractional knapsack (sort by value/weight ratio), and minimum spanning tree. Look for problems where local decisions don't affect future options.

What should I learn from easy problems?

Easy problems introduce core patterns that appear in harder problems. Master basic operations (iteration, conditionals, simple data structures), recognize common patterns (counting, searching, basic transformations), and practice explaining your thought process clearly.

What is the importance of time and space complexity analysis?

Complexity analysis is crucial because: 1) Interviewers always ask about it, 2) It helps you choose between approaches, 3) It demonstrates CS fundamentals. Always state both time and space complexity, and be prepared to explain how you derived them.

What is the optimal approach for "Maximum 69 Number"?

For this easy-level Math problem, the key is to identify the core pattern. Analyze the input constraints to determine acceptable time complexity. Consider whether greedy techniques can optimize the solution. Always handle edge cases and validate your approach with examples before coding.

🤖 Need help with this problem?