Find the first non-repeated character in a String 

This is a common question in coding interviews.
To find the first non-repeated character in a String, we can use multiple solutions.

Using LinkedHashMap

In questions where we need to count the number of times an element appears in a list or string, it is always best to use a HashMap. But, since the HashMap does not preserve the insertion order, and we need it here, we need to use a LinkedHashMap that does preserve the insertion order.

public class DevPrimer {

  public static void main(String[] args) {
    System.out.println(firstNonRepeatedCharacter("agcdefgchija"));
  }

  private static Character firstNonRepeatedCharacter(String str) {
    Map<Character, Integer> map = new LinkedHashMap<>();

    for (int i = 0; i < str.length(); i++) {
      if (map.containsKey(str.charAt(i))) {
        map.put(str.charAt(i), map.get(str.charAt(i)) + 1);
      } else {
        map.put(str.charAt(i), 1);
      }
    }

    for (Map.Entry<Character, Integer> entry : map.entrySet()) {
      if (entry.getValue() == 1) {
        return entry.getKey();
      }
    }

    return null;
  }
}

Output: d

Finds the first non-repeated character in a String in one pass 

Here, we are storing repeated and non-repeated characters separately:

public class DevPrimer {

  public static void main(String[] args) {
    System.out.println(firstNonRepeatedCharacter("agcdefgchija"));
  }

  private static Character firstNonRepeatedCharacter(String str) {
    Set<Character> repeating = new HashSet<>();
    List<Character> nonRepeating = new ArrayList<>();

    for (int i = 0; i < str.length(); i++) {
      if (repeating.contains(str.charAt(i))) {
        continue;
      }

      if (nonRepeating.contains(str.charAt(i))) {
        nonRepeating.remove((Character) str.charAt(i));
        repeating.add(str.charAt(i));
      } else {
        nonRepeating.add(str.charAt(i));
      }
    }

    return nonRepeating.get(0);
  }
}

Output: d

This is a classical time vs space trade-off.