subsequence

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);
    }
}

substring

We can run three nested loops, the outermost loop picks starting character, mid loop considers all characters on right of the picked character as ending character of substring. The innermost loop prints characters from currently picked starting point to picked ending point.

// C++ program to print all possible
// substrings of a given string

#include<bits/stdc++.h>
using namespace std;

// Function to print all sub strings
void subString(char str[], int n) 
{
    // Pick starting point
    for (int len = 1; len <= n; len++) 
    {    
        // Pick ending point
        for (int i = 0; i <= n - len; i++) 
        {
            //  Print characters from current
            // starting point to current ending
            // point.  
            int j = i + len - 1;            
            for (int k = i; k <= j; k++) 
                cout << str[k];

            cout << endl;
        }
    }
}

// Driver program to test above function
int main() 
{
    char str[] = "abc";
    subString(str, strlen(str));
    return 0;
}