Print all subsequences of a strin

Given a string, we have to find out all subsequences of it. 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 : a, b, c, ab, bc, ac, abc

Input : aaa
Output : a, aa, aaa

Explanation :

Step 1: Iterate over the entire String
Step 2: Iterate from the end of string 
        in order to generate different substring
        add the subtring to the list
Step 3: Drop kth character from the substring obtained 
        from above to generate different subsequence.
Step 4: if the subsequence is not in the list then recur.

Below is the implementation of the approach.

// Java Program to print all subsequence of a
// given string.
import java.util.HashSet;
public class Subsequence {
    
    // set to store all the subsequences
    static HashSet<String> st = new HashSet<>();
    // It computes all the subsequence of an string
    static void subsequence(String str)
    {
        // iterate over the entire string
        for (int i = 0; i < str.length(); i++) {
            
            // iterate from the end of the string
            // to generate substrings
            for (int j = str.length(); j > i; j--) {
                String sub_str = str.substring(i, j);
            
                if (!st.contains(sub_str))
                    st.add(sub_str);
                // drop kth character in the substring
                // and if its not in the set then recur
                for (int k = 1; k < sub_str.length() - 1; k++) {
                    StringBuffer sb = new StringBuffer(sub_str);
                    // drop character from the string
                    sb.deleteCharAt(k);
                    if (!st.contains(sb))
                        ;
                    subsequence(sb.toString());
                }
            }
        }
    }
    // Driver code
    public static void main(String[] args)
    {
        String s = "aabc";
        subsequence(s);
        System.out.println(st);
    }
}

Output:

[aa, a, ab, bc, ac, b, aac, abc, c, aab, aabc]

Disclaimer: This does not belong to TechCodeBit, its an article taken from the below
source and credits.
source and credits:http://www.geeksforgeeks.org/print-subsequences-string/
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rakesh

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