Generate consecutive substring from the string using recursion

Question

I am having a string let say ABCD I want to create all the subsets in such a way that they are consecutive like this:

`A` ,`AB`, `ABC`, `ABCD`, `B`, `BC`, `BCD`, `C`, `CD`, `D` 

AC, AD, BD etc should not be generated as they are not next to each other.

I tried to write the logic but now I am getting:

See the highlighted ones. Is it possible to generate consecutive subset using recursion?

Reason for using recursion is to understand how the recursion works so I am attempting all the questions using recursion only (without loops)

Code:

#include <iostream>
#include <vector>

using namespace std;

class Solution {
public:

void print_str(string s){
    for(int i=0; i<s.length();i++)
    {
        cout<<s[i];
    }
    cout<<endl;
}
void recur(string st, string ans, int prev, int n, int ind){

    print_str(ans);
    for(int i=prev; i<n;i++){
        recur(st, ans+st[i], i+1, n, ind+1);
    }

    }

};


int main()
{
    Solution sol;
    string st = "ABCD";
    sol.recur(st, "", 0, st.length(),0);
}

Answer 1

With recursion, you might do:

void print_seq(std::string_view s)
{
    if (s.size() > 1) print_seq(s.substr(0, s.size() - 1));
    std::cout << s << std::endl;   
}

void print_all(std::string_view s)
{
    print_seq(s);
    if (s.size() > 1) print_all(s.substr(1));
}

int main()
{
    print_all("ABCD");
}

Demo

Answer 2

From your rules, it can be said that once you started to append something to ans , you mustn't skip characters.

#include <iostream>
#include <string>

using namespace std;

class Solution {
public:

    void print_str(const string& s){
        cout<<s<<endl;
    }
    void recur(const string& st, string ans, int i, int n){
        if (i >= n){
            if (ans!="") print_str(ans); // print non-empty answer
        }else if(ans==""){ // when current answer is empty
            recur(st, ans+st[i], i+1, n); // start appending characters to the answer
            recur(st, ans, i+1, n); // or keep the answer empty
        }else{
            print_str(ans); // print non-empty answer
            recur(st, ans+st[i], i+1, n); // append a character (no choice to skip a character)
        }
    }

};


int main()
{
    Solution sol;
    string st = "ABCD";
    sol.recur(st, "", 0, st.length());
}

Output :

A
AB
ABC
ABCD
B
BC
BCD
C
CD
D