What is the time complexity of Increasing Decreasing String?

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

What data structures are used to solve Increasing Decreasing String?

This problem uses Hash Table, String, Counting. Understanding these concepts is essential for solving this Easy problem efficiently.

What are the key insights for solving Increasing Decreasing String?

The problem can be efficiently solved using a frequency count of each character. This allows for quick identification of the smallest and largest characters available at each step. Two iterations (ascending and descending order) are needed per round to pick the smallest and largest characters respectively.

Which companies ask LeetCode 1370. Increasing Decreasing String in interviews?

This problem is frequently asked at Akuna Capital.

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Increasing Decreasing String - LeetCode 1370 Solution

Increasing Decreasing String - Complete Solution Guide

Increasing Decreasing String is LeetCode problem 1370, 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 string s . Reorder the string using the following algorithm: Remove the smallest character from s and append it to the result. Remove the smallest character from s that is greater than the last appended character, and append it to the result. Repeat step 2 until no more characters can be removed. Remove the largest character from s and append it to the result. Remove the largest character from s that is smaller than the last appended character, and append it to the result. Repeat

Detailed Explanation

The problem asks you to reorder a given string `s` according to a specific algorithm. The algorithm iteratively removes the smallest and then the largest character from the string, appending them to the result. This process repeats until all characters are used. The order of smallest/largest characters is determined by their ASCII values. If multiple instances of the smallest or largest character exist, any occurrence can be chosen. The final reordered string is returned.

Solution Approach

The provided solutions use a frequency counting approach. First, they count the occurrences of each character in the input string. Then, they iterate through the character counts in ascending and descending order, adding characters to the result string as long as their count is greater than zero. This process is repeated until all characters have been added to the result string. The ascending pass picks the smallest available character, and the descending pass picks the largest.

Step-by-Step Algorithm

Create a frequency array (`counts` in Python/Java/C++, `count` in C) to store the count of each character (a-z).
Initialize an empty string `result` to store the reordered string.
Use a `while` loop that continues as long as the length of `result` is less than the length of the input string `s`.
Inside the `while` loop, iterate through the character counts from 'a' to 'z' (ascending order), appending characters to `result` if their count is greater than 0 and decrementing their count.
Next, iterate through the character counts from 'z' to 'a' (descending order), appending characters to `result` if their count is greater than 0 and decrementing their count.
Repeat steps 4 and 5 until all characters are processed.
Return the `result` string.

Key Insights

The problem can be efficiently solved using a frequency count of each character. This allows for quick identification of the smallest and largest characters available at each step.
Two iterations (ascending and descending order) are needed per round to pick the smallest and largest characters respectively.
A `while` loop continues until all characters from the input string are processed and added to the result string.

Complexity Analysis

Time Complexity: O(n)

Space Complexity: O(1)

Topics

This problem involves: Hash Table, String, Counting.

Companies

Asked at: Akuna Capital.

Increasing Decreasing String | LeetCode 1370

Easy

Hash Table String Counting

Time: O(n)Space: O(1)

Solution Code

Available in:

class Solution {
    public String sortString(String s) {
        int[] count = new int[26];
        for (char c : s.toCharArray()) {
            count[c - 'a']++;
        }

        StringBuilder result = new StringBuilder();
        while (result.length() < s.length()) {
            for (int i = 0; i < 26; i++) {
                if (count[i] > 0) {
                    result.append((char) (i + 'a'));
                    count[i]--;
                }
            }
            for (int i = 25; i >= 0; i--) {
                if (count[i] > 0) {
                    result.append((char) (i + 'a'));
                    count[i]--;
                }
            }
        }

        return result.toString();
    }
}

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Problem Info

Description

Problem 1370: Increasing Decreasing String - Detailed explanation and solution approaches.

Topics

Hash Table String Counting

Companies

Akuna Capital

Stats

Difficulty:Easy

Languages:4

Companies:1

Problem #:1370

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

7 steps

1
Create a frequency array (`counts` in Python/Java/C++, `count` in C) to store the count of each character (a-z).
2
Initialize an empty string `result` to store the reordered string.
3
Use a `while` loop that continues as long as the length of `result` is less than the length of the input string `s`.
4
Inside the `while` loop, iterate through the character counts from 'a' to 'z' (ascending order), appending characters to `result` if their count is greater than 0 and decrementing their count.
5
Next, iterate through the character counts from 'z' to 'a' (descending order), appending characters to `result` if their count is greater than 0 and decrementing their count.
6
Repeat steps 4 and 5 until all characters are processed.
7
Return the `result` string.

💡

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(1)

💪 Constant space - Uses same memory regardless of input!

📝

Why?

The dominant factor determining time complexity is the nested loops iterating through the frequency array. Although there are nested loops, the outer loop's number of iterations is bounded by the length of the input string and the inner loops have a fixed number of iterations (26). Hence the overall complexity is linear.

📖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

Using a Priority Queue

A priority queue (min-heap) could be used to store characters and their counts. The smallest character can be efficiently retrieved and removed. After processing the smallest characters, the priority queue could be reversed (using a max-heap or similar techniques) to process largest characters.

Time: O(n log n) due to insertion and deletion from the priority queue.Space: O(n) to store the priority queue.

Trade-offs: This approach is less efficient in time complexity than the counting method but might be more readable and easier to understand for some programmers. The counting method is generally preferred for this specific problem because of its better performance.

✨

Key Insights

1The problem can be efficiently solved using a frequency count of each character. This allows for quick identification of the smallest and largest characters available at each step.
2Two iterations (ascending and descending order) are needed per round to pick the smallest and largest characters respectively.
3A `while` loop continues until all characters from the input string are processed and added to the result string.

📚

Key Terms Explained

For Students

Hash Table

A data structure that maps keys to values for quick lookup. Like a dictionary!

String

A sequence of characters. Like a word or sentence in programming!

Edge Cases

Empty string: The code handles an empty string gracefully, returning an empty string.
String with only one character: The code correctly returns the single character string.

⚠️

Common Mistakes to Avoid

✗Forgetting to decrement the character count after adding a character to the `result` string.

✗Incorrectly handling the iteration order (ascending then descending) in each loop of the `while` loop, which is crucial for the algorithm's logic.

✅

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?

💼Data sorting and ordering: This algorithm can be adapted for tasks where data needs to be ordered in a specific pattern.

🏦String manipulation and processing: It's useful when processing strings where an order of specific characters based on their frequency is important.

💡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

Hash tables trade space for time. Before using one, consider if the problem constraints allow the extra memory. In interviews, always mention this trade-off.

🎯 Interview Tip

Don't rush. Explain your thought process clearly. Easy problems are often about communication.

⚠️ Watch Out

Don't forget to handle key collisions and null/undefined keys. Consider what happens with duplicate values.

👨‍💻 From Experience

This is one of the first problems I solved when learning Hash Table. It's a great foundation for more complex problems.

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: September 19, 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

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

@DevExpert92Jul 13

The key insight here is recognizing the pattern. Once you see it, the solution becomes straightforward.

@AlgoMasterJul 6

❓

Frequently Asked Questions

What are the trade-offs of using a hash table?

Hash tables provide O(1) average-case lookup, insertion, and deletion, making them ideal for frequency counting and duplicate detection. However, they use O(n) extra space and may degrade to O(n) time in worst-case scenarios with hash collisions. Always consider if the space trade-off is acceptable.

How do I handle string manipulation efficiently?

In languages with immutable strings (Java, Python, C#), use StringBuilder/list to avoid O(n) concatenation cost. Consider using character arrays for in-place modifications. For pattern matching, consider KMP or Rabin-Karp for O(n+m) complexity.

How should I approach easy-level algorithm problems?

Easy problems build foundational skills. Focus on: 1) Understanding the problem completely before coding, 2) Writing clean, readable code, 3) Handling all edge cases (empty input, single element, etc.), 4) Analyzing time and space complexity. Don't skip complexity analysis even for simple solutions.

How should I practice algorithm problems effectively?

Focus on understanding patterns rather than memorizing solutions. After solving a problem, review optimal solutions and understand the intuition. Group similar problems to recognize patterns. Practice explaining your approach out loud. Review problems after a few days to test retention.

What is the optimal approach for "Increasing Decreasing String"?

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

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