Скачать или смотреть Efficiently Find the First Repeated Character in a String in One Traversal

Learn how to find the first repeated character in a string efficiently with our step-by-step guide. We'll cover a Java implementation that meets the challenge without using any additional data structures.
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This video is based on the question https://stackoverflow.com/q/63297572/ asked by the user 'Shashank Singh' ( https://stackoverflow.com/u/13790873/ ) and on the answer https://stackoverflow.com/a/63297725/ provided by the user 'Henry' ( https://stackoverflow.com/u/1796579/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Introduction

As software developers, we often face challenges that require us to optimize our code for performance and efficiency. One common problem is finding the first repeated character in a string.

In this guide, we'll discuss a well-known algorithmic challenge: how to efficiently find the first repeated character without using any additional data structures while traversing the string just once.

Understanding the Problem

Given a string S consisting only of lowercase letters, the goal is to determine the first character that occurs more than once. The primary requirement is that the index of the character’s second occurrence should be the smallest across all repeated characters.

Example

For instance, consider the following examples:

For the input 'crg', there are no repeated characters, so the output should be -1 (indicating no repetition).

If the input is 'abcda', the first repeated character is 'a', which appears again at index 4.

The Challenge with Current Implementation

In the provided Java code, the approach attempts to store characters in a string and checks for duplicates using the indexOf method. However, there are a couple of issues that lead to incorrect outputs.

Let's analyze the code snippet:

[[See Video to Reveal this Text or Code Snippet]]

Issues Identified

Output Character Issue: The line char c = (char)-1 is problematic. It actually results in \uffff (which is not a valid character representation) and will display as ?. This misleads the output when there are no repeated characters.

Inefficient Character Collection: Using a String to collect unique characters and checking for the index is relatively inefficient, particularly as the length of the string increases.

Improving the Solution

Let's take a step back and come up with a more efficient approach that follows the constraints: no additional data structures and a single traversal of the string.

New Approach

Instead of using a string to collect characters, we can maintain a simple state using a character variable.

Revised Java Implementation

Here’s an updated version of the algorithm that addresses the issues identified previously:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Code

Outer Loop: This loop iterates through each character in the string.

Inner Loop: For each character in the outer loop, we check the subsequent characters to see if there’s a match.

Return Value: The first character that repeats is returned immediately. If there are no duplicates, \0 will be returned, indicating no repetition was found.

Conclusion

Finding the first repeated character in a string can be tricky. By understanding the common pitfalls of the problem and employing a straightforward approach, we can develop an efficient solution that meets the requirements of a single traversal without using additional data structures.

If you are faced with similar challenges or have any other coding problems, feel free to reach out or leave a comment below. Happy coding!