#1370

Easy

LeetCode Problem Workspace

Increasing Decreasing String

Reorder the string based on a specific algorithm using hash tables and string manipulation.

Problem Statement

You are given a string s. Your task is to reorder it using the following algorithm: first, find the smallest and largest characters in the string and append them to the result. Afterward, remove those characters from the string, then repeat the process for the remaining characters until all are used.

If the smallest or largest character appears more than once, you may choose any occurrence to append to the result. The goal is to return the resulting string after all characters have been reordered according to this rule.

Examples

Example 1

Input: s = "aaaabbbbcccc"

Output: "abccbaabccba"

After steps 1, 2 and 3 of the first iteration, result = "abc" After steps 4, 5 and 6 of the first iteration, result = "abccba" First iteration is done. Now s = "aabbcc" and we go back to step 1 After steps 1, 2 and 3 of the second iteration, result = "abccbaabc" After steps 4, 5 and 6 of the second iteration, result = "abccbaabccba"

Example 2

Input: s = "rat"

Output: "art"

The word "rat" becomes "art" after re-ordering it with the mentioned algorithm.

Constraints

1 <= s.length <= 500
s consists of only lowercase English letters.

Solution Approach

Count Character Frequencies

The first step is to count the frequency of each character in the string using a hash table (dictionary). This helps identify how often each character occurs, and it will guide the reordering process by knowing which characters should be placed in increasing or decreasing order.

Iterative Reordering

You will use two pointers or iterations: one for picking the smallest character and one for picking the largest character. Add characters to the result string alternately, moving through the available characters until the string is empty. After each round, update the frequencies to reflect the remaining characters.

Handle Multiple Occurrences

When characters appear multiple times, any occurrence can be chosen during the reordering process. The goal is to maintain the correct pattern while selecting characters from the hash table and adjusting their frequency for each step.

Complexity Analysis

Metric	Value
Time	Depends on the final approach
Space	Depends on the final approach

The time complexity of this solution depends on how efficiently the frequency counting and character extraction processes are handled. If you iterate through the string and use a hash table for counting, this can be done in O(n) time, where n is the length of the string. The space complexity is also O(n) due to the need for storing the frequency of each character in the hash table.

What Interviewers Usually Probe

Ability to implement hash table-based character counting efficiently.
Understanding of how to iteratively manipulate a string using counting and ordering techniques.
Knowledge of handling multiple occurrences of characters while maintaining the order pattern.

Common Pitfalls or Variants

Common pitfalls

Forgetting to handle characters with multiple occurrences correctly.
Not updating the frequency count properly after each character is added to the result.
Inefficient string manipulation that leads to higher time complexity.

Follow-up variants

Optimize the algorithm further for large strings with advanced counting methods.
Modify the solution to handle strings with uppercase letters or special characters.
Add constraints that require a specific space or time complexity.

FAQ

What is the approach for solving the Increasing Decreasing String problem?

The solution involves counting the frequency of each character and iterating through the string to reorder characters based on increasing and decreasing patterns using a hash table.

How does GhostInterview help with the Increasing Decreasing String problem?

GhostInterview helps by providing a structured approach, focusing on hash table usage, frequency counting, and string manipulation for efficient solutions.

What is the time complexity of the Increasing Decreasing String problem?

The time complexity is O(n), where n is the length of the string, due to the need to count characters and perform iterative reordering.

What are the common mistakes when solving the Increasing Decreasing String problem?

Common mistakes include forgetting to handle multiple occurrences of characters and inefficiently manipulating the string.

What are the key concepts in the Increasing Decreasing String problem?

The key concepts include hash tables for character counting, string manipulation, and iterative reordering based on frequency.

terminal

Solution

Solution 1: Counting + Simulation

First, we use a hash table or an array $cnt$ of length $26$ to count the number of occurrences of each character in the string $s$.

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class Solution:
    def sortString(self, s: str) -> str:
        cnt = Counter(s)
        cs = ascii_lowercase + ascii_lowercase[::-1]
        ans = []
        while len(ans) < len(s):
            for c in cs:
                if cnt[c]:
                    ans.append(c)
                    cnt[c] -= 1
        return "".join(ans)

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