Guess Number, Java

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//'main' method must be in a class 'Rextester'.
//Compiler version 1.8.0_111

import java.util.*;
import java.lang.*;

class Rextester
{  
    private static String target = "3536";
    public static void main(String args[])
    {
        System.out.println(guess());
    }
    
    private static int guessServer(String guess) { 
        System.out.println("GuessServer call: " + guess);
        int[] count = new int[6];
        int res = 0;
        for (int i = 0; i < 4; i++) count[target.charAt(i) - '1']++;
        for (int i = 0; i < 4; i++) if (count[guess.charAt(i) - '1']-- > 0) res++;
        return res;
    }

public static String guess() {
        StringBuilder sb = new StringBuilder();
        for (int i = 1; i < 6 && sb.length() < 4; i++) {
            int len = sb.length();
            while (sb.length() < 4) sb.append(i);
            int match = guessServer(sb.toString());
            sb.setLength(match);
        }
        while (sb.length() < 4) sb.append(6);
        return sb.toString();
    }
}

[ + ] Show input
Compilation time: 0.83 sec, absolute running time: 0.2 sec, cpu time: 0.18 sec, memory peak: 18 Mb, absolute service time: 1,04 sec