Find first non-repeating character from a String

Carvia Tech | December 04, 2019 | 1 min read | 273 views

Approach

Use a LinkedHashmap to store frequency map of each character. LinkedHashmap keeps the items in the order of their insertion.
Iterate over all the input String one character at a time.
Filter only those entries that have frequency as 1.
Find the fist entry and get its key. This character has not been repeated from given input.

Traditional approach

String str = "zzzzzbbbccccddehhhhiii";
int[] countingArray = new int[128];
str.chars().forEach(value -> countingArray[value]++);
int nonRepeatingCharAsInt = 0;
for (int i = 0; i < countingArray.length; i++) {
    if (countingArray[i] == 1) {
        nonRepeatingCharAsInt = i;
        break;
    }
}
System.out.println("character = " + Character.valueOf((char) nonRepeatingCharAsInt));

using Java 8 Streams

import java.util.LinkedHashMap;
import java.util.Optional;
import java.util.stream.Collectors;

import static java.util.function.Function.identity;

public class Utils {

    private Optional<Character> findFirstNonRepeatingLetter(String s) {
        final Optional<Character> optionalCharacter = s.chars()
                .mapToObj(i -> (char) i)
                .collect(Collectors.groupingBy(identity(), LinkedHashMap::new, Collectors.counting()))
                .entrySet().stream()
                .filter(entry -> entry.getValue() == 1L)
                .map(entry -> entry.getKey())
                .findFirst();
        return optionalCharacter;
    }
}

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