Find longest substring without repeating characters

Question

Given a string S of length N find longest substring without repeating characters.

Example:

Input:

Answer 1

import java.util.HashMap;
import java.util.HashSet;

public class SubString {
    public static String subString(String input) {

        String longesTillNOw = "";

        String longestOverAll = "";
        HashMap chars = new HashMap<>();
        char[] array=input.toCharArray();
        int start=0;
        for (int i = 0; i < array.length; i++) {
            char charactor = array[i];
            if (chars.containsKey(charactor) ) {
                start=chars.get(charactor)+1;
                i=start;
                chars.clear();
                longesTillNOw = "";
            } else {
                chars.put(charactor,i);
                longesTillNOw = longesTillNOw + charactor;
                if (longesTillNOw.length() > longestOverAll.length()) {
                    longestOverAll = longesTillNOw;
                }
            }
        }
        return longestOverAll;

    }

    public static void main(String[] args) {
        String input = "stackoverflowabcdefghijklmn";
        System.out.println(subString(input));
    }
}