Print all Subsequences of String which Start with Vowel and End with Consonant.

Given a string return all possible subsequences which start with vowel and end with consonant. A String is a subsequence of a given String, that is generated by deleting some character of a given string without changing its order.
Examples:

Input : 'abc'
Output : ab, ac, abc

Input : 'aab'
Output : ab, aab

Question Source: Yatra.com Interview Experience | Set 7

Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.

Explanation of the Algorithm:

Step 1: Iterate over the entire String
Step 2: check if the ith character for vowel
Step 3: If true iterate the string from the end,
        if false move to next iteration
Step 4: check the jth character for consonent
        if false move to next iteration 
        if true perform the following 
Step 5: Add the substring starting at index i and
        ending at index j to the hastset.
Step 6: Iterate over the substring drop each character 
        and recur to generate all its subString

// Java Program to generate all the subsequence 
// starting with vowel and ending with consonant. 
import java.util.HashSet; 
  
public class Subsequence { 
  
    // Set to store all the subsequences 
    static HashSet<String> st = new HashSet<>(); 
  
    // It computes all the possible substring that 
    // starts with vowel and end with consonent 
    static void subsequence(String str) 
    { 
        // iterate over the entire string 
        for (int i = 0; i < str.length(); i++) { 
          
            // test ith character for vowel 
            if (isVowel(str.charAt(i))) { 
          
                // if the ith character is vowel 
                // iterate from end of the string 
                // and check for consonant. 
                for (int j = (str.length() - 1); j >= i; j--) { 
                      
                    // test jth character for consonant. 
                    if (isConsonant(str.charAt((j)))) { 
                      
                        // once we get a consonant add it to  
                        // the hashset 
                        String str_sub = str.substring(i, j + 1); 
                        st.add(str_sub); 
  
                        // drop each character of the substring 
                        // and recur to generate all subsquence 
                        // of the substring 
                        for (int k = 1; k < str_sub.length() - 1; k++) { 
                            StringBuffer sb = new StringBuffer(str_sub); 
                            sb.deleteCharAt(k); 
                            subsequence(sb.toString()); 
                        } 
                    } 
                } 
            } 
        } 
    } 
  
    // Utility method to check vowel 
    static boolean isVowel(char c) 
    { 
        return (c == 'a' || c == 'e' || c == 'i' || c == 'o'
                                              || c == 'u'); 
    } 
  
    // Utility method to check consonant 
    static boolean isConsonant(char c) 
    { 
        return !(c == 'a' || c == 'e' || c == 'i' || c == 'o'
                                              || c == 'u'); 
    } 
  
    // Driver code 
    public static void main(String[] args) 
    { 
        String s = "xabcef"; 
        subsequence(s); 
        System.out.println(st); 
    } 
} 

Output:

[ef, ab, ac, aef, abc, abf, af, acf, abcef, abcf, acef, abef]

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