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Recursion for Coding Interviews in Java

Solution Review: Let's find the total number of vowels!

Explore how recursion can be applied to count vowels in a string. Understand the structure of recursive methods with base and recursive cases, and see how to process each character effectively. This lesson helps you improve your recursive problem-solving skills with strings in Java.

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Solution: How many vowels are there?

Java
class ChallengeClass {
    public static int totalVowels(String text, int len, int index) {
        int count = 0;
        if (len == 0) {
            return 0;
        }
        char single = Character.toUpperCase(text.charAt(index));
        if (single == 'A' || single == 'E' || single == 'I' || single == 'O' || single == 'U') {
            count++;
        }
        return count + totalVowels(text, len-1, index+1);
    }
    public static int callTotalVowels(String text) {
        return totalVowels(text, text.length(), 0);
    }
    public static void main( String args[] ) {
        String string1 = "Hello world";
        String string2 = "STR";
        String string3 = "AEIOUaeiouSs";
        int result1 = callTotalVowels(string1);
        int result2 = callTotalVowels(string2);
        int result3 = callTotalVowels(string3);
        System.out.println( "Total number of vowels in " + string1 + " are = " + result1);
        System.out.println( "Total number of vowels in " + string2 + " are = " + result2);
        System.out.println( "Total number of vowels in " + string3 + " are = " + result3);
    }
}

Understanding the Code

Every recursive code has two methods; the recursive method and the main method.

Driver Method

The recursive method is called within the driver method. Let’s first look at what it does-from line 21 to line 31.

  • There are three calls made to the recursive method. Each call, prints the given string and the total number of vowels within it.
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