Find The First Non Repeated Character In A String : Technical Interview Question

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

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 ;    }}