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First non-repeating character in a string

Write a program to find the first non-repeating character of a string.

The easiest solution to this would be to take each character and compare it with rest of the characters. If there is no match, then that is the first non-repeating character.

But you realize the problem with this solution, right. It is very in-efficient. It takes n*n iteration.

Another solution is - create a hashmap of characters and their frequencies
  •       browse through all characters in the string
    •       if the character is present in map, increment its count
    •       if not present add it to map with count as 1
Next browse through characters of the string again
  • if the count is >1, skip to next character
  • if the count is 1, return that character - we have found our first non-repeating character. 
Here is the complete program


import java.util.HashMap;
import java.util.Scanner;

public class NonRepeating {
    char firstNonRepeating(String str){
        char [] arr = str.toCharArray();
        HashMap<Character,Integer> map = new HashMap<>();
        for (char ch :arr){/*create a map of characters and frequencies*/
            if(map.containsKey(ch)){
                int count = map.get(ch);
                map.replace(ch,count,count+1);
            }else{
                map.put(ch,1);
            }
        }
        for(char ch:arr){
            int frequency = map.get(ch);
            if(frequency==1)
                return ch;
        }
        return (char) -1;
    }
    public static void main(String args[]){
        Scanner scanner = new Scanner(System.in);
        System.out.println("Enter the string:");
        String str = scanner.nextLine();
        NonRepeating obj = new NonRepeating();
        char nonRepChr = obj.firstNonRepeating(str);
        if((int)nonRepChr!=-1)
            System.out.println("The first non-repeating character is "+nonRepChr);
        else
            System.out.println("There are no such characters");
    }
}
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