CC-17 : Find first non repeating character.

Description:

Given input String str, find and return the first non-repeating character in str.

Test cases and expected outputs:

Input Parameters	Expected outputs
The input string is: Hello Hello World	The first non repeating char is: W
The original string is: The lazy brown fox	The first non repeating char is: T
The original string is: AAA	There is no non repeating character

Pseudocode:

Iterate through chars using for loop using idx as loop variable, starting from index 0 and incrementing idx till we reach chars.length-1:

The java method should accept following input parameters: str (String).

Initialize char array chars=str.toCharArray().

Initialize char array counts = new char[256]. We will use this array to store counts of characters found.

When we cast chars[idx] as integer, it coverts to a unique integer for each character. For example if chars[idx]=’H’, when we cast it as an integer it maps to 72. We can use 72 as the index of counts and increment counts[72] whenever we find character ‘H’ in chars. We can do this similarly for all other characters in chars.

Code for above is: counts[chars[idx]]++.

At the end of above loop, counts store the repetition counts for all characters in chars.

Iterate through chars using for loop using idx as loop variable, starting from index 0 and incrementing idx till we reach chars.length-1:

Check counts[chars[idx] for each iteration, if it is equal to 1, this is the first non-repeating char in chars, Return this chars[idx] from the program.

If the above loop completes and we have not returned from the program with the first non-repeating character it means no such non repeating character exists in str. Return “” blank string from the program.

Code:

public char stringFirstNonRepeatingChar(String str) {
	char[] chars=str.toCharArray();
	char[] counts = new char[256];
	for (int idx=0; idx < str.length(); idx++) {
		System.out.println(chars[idx] + " " + (int)chars[idx]);
		counts[chars[idx]]++;
	}
	for (int idx=0; idx < str.length(); idx++) {
		if (counts[chars[idx]]==1) {
			return chars[idx];
		};
	}
	
	return '\u0000';
}

Click here to download and run code and test cases !

Below fully running code can be copied and run on Eclipse or other Java IDEs. Refer the classname in code below. If the class name below is "A", save the code below to a file named A.java before running it. Be sure to try your own test cases to enhance your understanding ! You can also tweak the code to optimize or add enhancements and custom features.

public class StringFirstNonRepeating {
	
public char stringFirstNonRepeatingChar(String str) {
	char[] chars=str.toCharArray();
	char[] counts = new char[256];
	for (int idx=0; idx < str.length(); idx++) {
		System.out.println(chars[idx] + " " + (int)chars[idx]);
		counts[chars[idx]]++;
	}
	for (int idx=0; idx < str.length(); idx++) {
		if (counts[chars[idx]]==1) {
			return chars[idx];
		};
	}
	
	return '\u0000';
}
	
public static void main(String[] args) {
	StringFirstNonRepeating sp=new StringFirstNonRepeating();
	char retStr;
		
	try {
	String str1="Hello Hello World";
	System.out.println("\n\nThe original string is: "+str1);
	retStr=sp.stringFirstNonRepeatingChar(str1);
	System.out.println("The first non repeating char is: "+retStr);
			
	str1="The lazy brown fox";
	System.out.println("\n\nThe original string is: "+str1);
	retStr=sp.stringFirstNonRepeatingChar(str1);
	System.out.println("The first non repeating char is: "+retStr);
			
	str1="AAA";
	System.out.println("\n\nThe original string is: "+str1);
	retStr=sp.stringFirstNonRepeatingChar(str1);
	System.out.println("The first non repeating char is: "+retStr);
			
	str1="HaHa";
	System.out.println("\n\nThe original string is: "+str1);
	retStr=sp.stringFirstNonRepeatingChar(str1);
	System.out.println("The first non repeating char is: "+retStr);
			
			
	}catch (Exception exception) {
	System.out.print("Exception,"+ exception);
	exception.printStackTrace();
				
	}
}

}