Find the first non repeated character in a string : Technical interview question

In written exams or in technical interview  of Amazon , you can easily find this question . So in order to help you for the preparation of interview , we are answering this question in this post .

First we need to understand the question , we need to write an algorithm for  the first non repeated character in a string , for example :

Read Also :   Count number of words in the String

If the word "stress" is input  then it should print  't'   as output .

If the word "teeter" is input  then it should print  'r'   as output .

Now , we should understand the pseudo algorithm or logic to achieve this task , code is given in the end of this post .

Logic

As we know a non repeated character occurs only once in the string , so if we store the number of times
each alphabet appears in the string , it would help us identifying which characters are non repeated characters in the string .So we need to scan the whole string and determine the final counts of each character . Now scan the final values of each character in the string , the first character in the string with final count 1 is the first non repeated character in the string.

Pseudo Algorithm

1.   First create the  character count hash table .
 
          For each character
            If there is no value stored in the character
                     set it to 1 .
            else
                     increment the value of the character by 1 .

2.  Scan the string
           For each character
           return character if the count in hash table is 1 .
           If no character have count 1 , return null

Read Also :   Count total number of times each alphabet appears in the String

Code :

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


public class FirstNonRepeated {
    
    public static void main(String[] args)
    {
        // TODO Auto-generated method stub
        
        System.out.println(" Please enter the input string :" );
        Scanner in = new Scanner (System.in);
        String s=in.nextLine();
        char c=firstNonRepeatedCharacter(s);
        System.out.println("The first non repeated character is :  " + c);
    }
    
    public static Character firstNonRepeatedCharacter(String str)
    {
        HashMap<Character,Integer>  characterhashtable= 

                     new HashMap<Character ,Integer>();
        int i,length ;
        Character c ;
        length= str.length();  // Scan string and build hash table
        for (i=0;i < length;i++)
        {
            c=str.charAt(i);
            if(characterhashtable.containsKey(c))
            {
                // increment count corresponding to c
                characterhashtable.put(  c ,  characterhashtable.get(c) +1 );
            }
            else
            {
                characterhashtable.put( c , 1 ) ;
            }
        }
        // Search characterhashtable in in order of string str
        
        for (i =0 ; i < length ; i++ )
        {
            c= str.charAt(i);
            if( characterhashtable.get(c)  == 1 )
            return c;
        }
        return null ;
    }
}