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class Solution {
    public String solution(String my_string, int m, int c) {
        StringBuilder answer = new StringBuilder();

        int startIndex = c - 1;
        while (startIndex < my_string.length()) {
            answer.append(my_string.charAt(startIndex));
            startIndex += m;
        }

        return answer.toString();
    }
}

 

import java.util.*;

class Solution {
    public String solution(String my_string, int m, int c) {
        List<String> result = new ArrayList<>();
        for (int i = 0; i < my_string.length(); i += m) {
            String sub = my_string.substring(i, i + m);
            result.add(sub);
        }
        
        String answer = "";
        for (String str : result) {
            answer += str.substring(c-1, c);
        }
        
        return answer;
    }
}

 

class Solution {

    public String solution(String my_string, int m, int c) {
        String answer = "";

        for (int i = c - 1; i < my_string.length(); i += m) {
            answer += my_string.charAt(i);
        }
        return answer;
    }
}

import java.util.stream.*;

class Solution {
    public String solution(String myString, int m, int c) {
        return IntStream.range(0, myString.length())
            .filter(i -> i % m == c - 1)
            .mapToObj(i -> String.valueOf(myString.charAt(i)))
            .collect(Collectors.joining());
    }
}