Java Program to Find the First Non-repeated Character in a String

This is the Java Program to Find the First non-repeated Character in a String

Problem Description

Given a string of characters, find the first non-repeated character in the string.

Example:

String x = “Hi this is Marry”
Output = ‘H’

Problem Solution

Make two new arrays to store the frequency of each character and the indexes of there occurrences. Iterate through the string to fill the two arrays. Finally, re-iterate through the new arrays, to find the index of the minimum index of the characters whose frequency are 1.

Program/Source Code

Here is the source code of the Java Program to Find the First non-repeated Character in a String. The program is successfully compiled and tested using IDE IntelliJ Idea in Windows 7. The program output is also shown below.

```
 
```

// Java Program to Find the First non-repeated Character in a String.

```
 
```
```
import java.io.BufferedReader;
```
```
import java.io.IOException;
```
```
import java.io.InputStreamReader;
```
```
 
```
```
public class NonRepeatedCharacter {
```

    // Function to find the first non-repeating character

    static int firstNonRepeatingCharacter(String str){

        int[] arrayCount = new int[256];

        int[] arrayIndex = new int[256];

```
        int i;
```
```
        for(i=0; i<str.length(); i++){
```

            arrayCount[str.charAt(i)]++;

            arrayIndex[str.charAt(i)] = i;

```
        }
```
```
        int index = Integer.MAX_VALUE;
```
```
        for(i=0; i<256; i++){
```

            if(arrayCount[i] == 1 && arrayIndex[i] < index)

```
                index = arrayIndex[i];
```
```
        }
```
```
        return index;
```
```
    }
```
```
    // Function to read user input
```

    public static void main(String[] args) {

        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

```
        String str;
```

        System.out.println("Enter the string");

```
        try {
```
```
            str = br.readLine();
```
```
        } catch (IOException e) {
```

            System.out.println("An I/O error occurred");

```
            return;
```
```
        }
```

        int index = firstNonRepeatingCharacter(str);

```
        if(index < str.length())
```

            System.out.println("First Non Repeating Character is "

                                              + str.charAt(index));

```
        else
```

            System.out.println("Each character is repeated");

```
    }
```
```
}
```

Program Explanation

1. In function firstNonRepeatingCharacter(), two arrays arrayCount and arrayIndex are created.
2. The loop for(i=0; i<str.length(); i++) iterates through the string and counts the frequency of each character in arrayCount array and the index of the last occurence of a character in arrayIndex array.
3. The loop for(i=0; i<256; i++) iterates through the two arrays arrayCount and arrayIndex.
4. The condition if(arrayCount[i] == 1 && arrayIndex[i] < index) checks if the frequency of character is 1 and the second part looks for first non-repeating character.

Time Complexity: O(n) where n is the length of the string.

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Runtime Test Cases

 
Case 1 (Positive Test Case):
 
Enter the string
Hi this is Marry
First Non-Repeating Character is H
 
Case 2 (Positive Test Case - another example):
 
Enter the string
well well finally you are here
First Non-Repeating Character is f
 
Case 3 (Negative Test Case):
 
Enter the string
WellWell
Each character is repeated

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