demo, Java - rextester

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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
{  
    public static Character firstNonRepeatedCharacter(String str) {
        if (str == null) return null;

LinkedHashMap<Character, Integer> map = new LinkedHashMap<>();
        for (int i = 0; i < str.length(); i++) {
            Integer cnt = map.get(str.charAt(i));
            map.put(str.charAt(i), cnt == null ? 1 : cnt.intValue() + 1);
        }
 
        // now just walk the list, in insertion order, and return the first key
        // whose value is 1 (indicating that it never got repeated)
        for (Map.Entry<Character, Integer> entry : map.entrySet()) {
            if (entry.getValue() == 1) {
                return entry.getKey();
            }
        }

// return null if no such key appearing only once occurred in the string
        return null;
    }

public static void main(String args[])
    {
        Character result = firstNonRepeatedCharacter("abcdcd"); // --> 'a'
        System.out.println("abcdcd -> " + result);
        result = firstNonRepeatedCharacter("cbcd"); // --> 'b'
        System.out.println("cbcd -> " + result);
        result = firstNonRepeatedCharacter("cdcd"); // --> null
        System.out.println("cdcd -> " + result);
    }
}

[ + ] Show input
Compilation time: 0.73 sec, absolute running time: 0.13 sec, cpu time: 0.11 sec, memory peak: 18 Mb, absolute service time: 0,86 sec