Print all subsequences of a string using ArrayList

Given a string str, the task is to print all the sub-sequences of str.
A subsequence is a sequence that can be derived from another sequence by deleting some or no elements without changing the order of the remaining elements.

Examples:

Input: str = “abc”
Output: a b ab c ac bc abc

Input: str = “geek”
Output: g e ge e ge ee gee k gk ek gek ek gek eek geek

Approach: Write a recursive function that prints every sub-sequence of the sub-string starting from the second character str[1, n – 1] after appending the first character of the string str[0] in the beginning of every sub-sequence. Terminating condition will be when the passed string is empty, in that case the function will return an empty arraylist.

Below is the implementation of the above approach:

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// Java implementation of the approach
import java.util.ArrayList;
  
public class GFG {
  
    // Utility function to print the contents
    // of the ArrayList
    static void printArrayList(ArrayList<String> arrL)
    {
        arrL.remove("");
        for (int i = 0; i < arrL.size(); i++)
            System.out.print(arrL.get(i) + " ");
    }
  
    // Function to returns the arraylist which contains
    // all the sub-sequences of str
    public static ArrayList<String> getSequence(String str)
    {
  
        // If string is empty
        if (str.length() == 0) {
  
            // Return an empty arraylist
            ArrayList<String> empty = new ArrayList<>();
            empty.add("");
            return empty;
        }
  
        // Take first character of str
        char ch = str.charAt(0);
  
        // Take sub-string starting from the
        // second character
        String subStr = str.substring(1);
  
        // Recurvise call for all the sub-sequences 
        // starting from the second character
        ArrayList<String> subSequences = 
                              getSequence(subStr);
  
        // Add first character from str in the beginning
        // of every character from the sub-sequences
        // and then store it into the resultant arraylist
        ArrayList<String> res = new ArrayList<>();
        for (String val : subSequences) {
            res.add(val);
            res.add(ch + val);
        }
  
        // Return the resultant arraylist
        return res;
    }
  
    // Driver code
    public static void main(String[] args)
    {
        String str = "geek";
        printArrayList(getSequence(str));
        // System.out.print(getSequence(str));
    }
}

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Output:

g e ge e ge ee gee k gk ek gek ek gek eek geek

Alternate Solution: One by one fix characters and recursively generate all subsets starting from them.

Java

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// Java implementation of the approach
public class sub_sequence {
  
    // Function to print all the sub-sequences
    // of a string
    public static void printSubSeq(String sub, 
                                  String ans)
    {
  
        if (sub.length() == 0) {
            System.out.print("" + ans + " ");
            return;
        }
  
        // First character of sub
        char ch = sub.charAt(0);
  
        // Sub-string starting from second
        // character of sub
        String ros = sub.substring(1);
  
        // Excluding first character
        printSubSeq(ros, ans);
  
        // Including first character
        printSubSeq(ros, ans + ch);
    }
  
    // Driver code
    public static void main(String[] args)
    {
        String str = "abc";
        printSubSeq(str, "");
    }
}

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C#

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// C# implementation of the approach
using System;
  
class GFG 
  
// Function to print all the  
// sub-sequences of a string 
public static void printSubSeq(string sub, 
                               string ans) 
    if (sub.Length == 0) 
    
        Console.Write("" + ans + " "); 
        return
    
  
    // First character of sub 
    char ch = sub[0]; 
  
    // Sub-string starting from second 
    // character of sub 
    string ros = sub.Substring(1); 
  
    // Excluding first character 
    printSubSeq(ros, ans); 
  
    // Including first character 
    printSubSeq(ros, ans + ch); 
  
// Driver code 
public static void Main() 
    string str = "abc";
    printSubSeq(str, "") ;
}
  
// This code is contributed by Ryuga

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Output:

c b bc a ac ab abc


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Improved By : AnkitRai01