Find first non-repeating character from a String

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


Approach

  1. Use a LinkedHashmap to store frequency map of each character. LinkedHashmap keeps the items in the order of their insertion.

  2. Iterate over all the input String one character at a time.

  3. Filter only those entries that have frequency as 1.

  4. 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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