Rearrange String: Move Vowels to the Beginning While Keeping Order

 

Problem Statement

Given a string s, rearrange the characters such that all vowels appear at the beginning, while maintaining the relative order of the consonants. The order of vowels should also remain the same as in the original string.

Example 1

Input: "automation"
Output: "auaotmtn"

Example 2

Input: "hello"
Output: "eo hll"

Example 3

Input: "java"
Output: "aa jv"

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Solution Approach

1. Extract vowels in order.

2. Extract consonants in order.

3. Concatenate vowels + consonants to form the result.

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You can find the video here

Java Solution

public class MoveVowelsToLeft {

    public static String moveVowelsToLeft(String s) {

        StringBuilder vowels = new StringBuilder();

        StringBuilder consonants = new StringBuilder();

        

        for (char c : s.toCharArray()) {

            if (isVowel(c)) {

                vowels.append(c);

            } else {

                consonants.append(c);

            }

        }

        return vowels.append(consonants).toString();

    }

    private static boolean isVowel(char c) {

        return "AEIOUaeiou".indexOf(c) != -1;

    }

    public static void main(String[] args) {

        System.out.println(moveVowelsToLeft("automation")); // Output: auaotmtn

        System.out.println(moveVowelsToLeft("hello"));      // Output: eo hll

        System.out.println(moveVowelsToLeft("java"));       // Output: aa jv

    }

}

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Time Complexity Analysis

• O(n), where n is the length of the string (single pass to classify characters, another pass to concatenate).

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