NonRepeating1.java

import java.io.IOException;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;

/**
 * Java Program to find first duplicate, non-repeated character in a String.
 * It demonstrate three simple example to do this programming problem.
 *
 * @author Javarevisited
 */
public class Programming {
    
    /*
     * Using LinkedHashMap to find first non repeated character of String
     * Algorithm :
     *            Step 1: get character array and loop through it to build a 
     *                    hash table with char and their count.
     *            Step 2: loop through LinkedHashMap to find an entry with 
     *                    value 1, that's your first non-repeated character,
     *                    as LinkedHashMap maintains insertion order.
     */
    public static char getFirstNonRepeatedChar(String str) {
        Map<Character,Integer> counts = new LinkedHashMap<>(str.length());
        
        for (char c : str.toCharArray()) {
            counts.put(c, counts.containsKey(c) ? counts.get(c) + 1 : 1);
        }
        
        for (Entry<Character,Integer> entry : counts.entrySet()) {
            if (entry.getValue() == 1) {
                return entry.getKey();
            }
        }
        throw new RuntimeException("didn't find any non repeated Character");
    }


    /*
     * Finds first non repeated character in a String in just one pass.
     * It uses two storage to cut down one iteration, standard space vs time
     * trade-off.Since we store repeated and non-repeated character separately,
     * at the end of iteration, first element from List is our first non
     * repeated character from String.
     */
    public static char firstNonRepeatingChar(String word) {
        Set<Character> repeating = new HashSet<>();
        List<Character> nonRepeating = new ArrayList<>();
        for (int i = 0; i < word.length(); i++) {
            char letter = word.charAt(i);
            if (repeating.contains(letter)) {
                continue;
            }
            if (nonRepeating.contains(letter)) {
                nonRepeating.remove((Character) letter);
                repeating.add(letter);
            } else {
                nonRepeating.add(letter);
            }
        }
        return nonRepeating.get(0);
    }


    /*
     * Using HashMap to find first non-repeated character from String in Java.
     * Algorithm :
     * Step 1 : Scan String and store count of each character in HashMap
     * Step 2 : traverse String and get count for each character from Map.
     *          Since we are going through String from first to last character,
     *          when count for any character is 1, we break, it's the first
     *          non repeated character. Here order is achieved by going
     *          through String again.
     */
    public static char firstNonRepeatedCharacter(String word) {
        HashMap<Character,Integer> scoreboard = new HashMap<>();
        // build table [char -> count]
        for (int i = 0; i < word.length(); i++) {
            char c = word.charAt(i);
            if (scoreboard.containsKey(c)) {
                scoreboard.put(c, scoreboard.get(c) + 1);
            } else {
                scoreboard.put(c, 1);
            }
        }
        // since HashMap doesn't maintain order, going through string again
        for (int i = 0; i < word.length(); i++) {
            char c = word.charAt(i);
            if (scoreboard.get(c) == 1) {
                return c;
            }
        }
        throw new RuntimeException("Undefined behaviour");
    }

}


Read more: https://javarevisited.blogspot.com/2014/03/3-ways-to-find-first-non-repeated-character-String-programming-problem.html#ixzz6DCR2JiKb