First non repeating character

Created on: Jan 15, 2025

Find first non-repeating character of given string.

Input: s = “geeksforgeeks”
Output: ‘f’
Explanation: ‘f’ is the first character in the string which does not repeat.


Input: s = “racecar”
Output: ‘e’
Explanation: ‘e’ is the only character in the string which does not repeat.

public class Solution {
    public static char findFirst(String input) {
       return null;
    }

    public static void main(String[] args) {
        String str = "ababdc";
        char c = findFirst(str);
        System.out.println(c);
    }
}

Solution using Linked hash set

public static char findFirst(String input) {
        Set<Character> set = new LinkedHashSet<>();
        for (int i = 0; i < input.length(); i++) {
            if(set.contains(input.charAt(i))){
                set.remove(input.charAt(i));
            }else {
                set.add(input.charAt(i));
            }
        }
        return set.iterator().next();
    }

Solution using frequency array

import java.util.Arrays;

public class Solution {
    public static char findFirst(String input) {
        int[] freq = new int[26];
        Arrays.fill(freq, 0);

        return findFirst(input, freq);
    }

    private static char findFirst(String input, int[] freq){
        for(int i=0; i<input.length(); i++){
            freq[input.charAt(i)-'a']++;
        }

        for(int i=0; i<input.length(); i++){
            if(freq[input.charAt(i)-'a']==1){
                return input.charAt(i);
            }
        }
        return '$';
    }

    public static void main(String[] args) {
        String str = "ababdc";
        char c = findFirst(str);
        System.out.println(c);
    }
}

Reference

geeksforgeeks