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K’th Non-repeating Character

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Given a string str of length n (1 <= n <= 106) and a number k, the task is to find the kth non-repeating character in the string.

Examples: 

Input : str = geeksforgeeks, k = 3
Output : r
Explanation: First non-repeating character is f, second is o and third is r.

Input : str = geeksforgeeks, k = 2
Output : o

Input : str = geeksforgeeks, k = 4
Output : Less than k non-repeating characters in input.

This problem is mainly an extension of the First non-repeating character problem.

K’th Non-repeating Character using Nested Loop:

A Simple Solution is to run two loops. Start traversing from the left side. For every character, check if it repeats or not. If the character doesn’t repeat, increment the count of non-repeating characters. When the count becomes k, return the character.

Below is the implementation of the above idea:

C++

// Include all standard libraries
#include <bits/stdc++.h>
using namespace std;
 
// Define a function to find the kth non-repeating character
// in a string
char kthNonRepeatingChar(string str, int k)
{
    // Initialize count and result variables to 0 and null
    // character, respectively
    int count = 0;
    char result = '\0';
 
    // Loop through each character in the string
    for (int i = 0; i < str.length(); i++) {
        // Assume that the current character does not repeat
        bool repeating = false;
 
        // Loop through the rest of the string to check if
        // the current character repeats
        for (int j = 0; j < i; j++) {
                if(str[j]==str[i])
               {
                repeating = true;
               }
        }
                    
        for (int j = i + 1; j < str.length(); j++) {
            if (str[i] == str[j]) {
                // If the current character repeats, set the
                // repeating flag to true and exit the loop
                repeating = true;
            }
        }
 
        // If the current character does not repeat,
        // increment the count of non-repeating characters
        if (!repeating) {
            count++;
            // If the count of non-repeating characters
            // equals k, set the result variable to the
            // current character and exit the loop
            if (count == k) {
                result = str[i];
                break;
            }
        }
    }
 
    // Return the result variable
    return result;
}
 
// Define the main function to test the kthNonRepeatingChar
// function
int main()
{
    // Define an example string and value of k
    string str = "geeksforgeeks";
    int k = 3;
 
    // Call the kthNonRepeatingChar function with the
    // example string and value of k
    char result = kthNonRepeatingChar(str, k);
 
    // Check if the result variable contains a non-null
    // character and print the appropriate message
    if (result == '\0') {
        cout << "There is no kth non-repeating character "
                "in the string.\n";
    }
    else {
        cout << "The " << k
             << "th non-repeating character in the string "
                "is "
             << result << ".\n";
    }
 
    // End the program
    return 0;
}

                    

Java

// Import required libraries
import java.util.*;
 
// Define a class to find the kth non-repeating character
// in a string
public class Main {
    public static char kthNonRepeatingChar(String str,
                                           int k)
    {
        // Initialize count and result variables to 0 and
        // null character, respectively
        int count = 0;
        char result = '\0';
 
        // Loop through each character in the string
        for (int i = 0; i < str.length(); i++) {
            // Assume that the current character does not
            // repeat
            boolean repeating = false;
 
            // Loop through the rest of the string to check
            // if the current character repeats
            for (int j = i + 1; j < str.length(); j++) {
                if (str.charAt(i) == str.charAt(j)) {
                    // If the current character repeats, set
                    // the repeating flag to true and exit
                    // the loop
                    repeating = true;
                    break;
                }
            }
 
            // If the current character does not repeat,
            // increment the count of non-repeating
            // characters
            if (!repeating) {
                count++;
                // If the count of non-repeating characters
                // equals k, set the result variable to the
                // current character and exit the loop
                if (count == k) {
                    result = str.charAt(i);
                    break;
                }
            }
        }
 
        // Return the result variable
        return result;
    }
 
    // Define the main function to test the
    // kthNonRepeatingChar function
    public static void main(String[] args)
    {
        // Define an example string and value of k
        String str = "geeksforgeeks";
        int k = 3;
 
        // Call the kthNonRepeatingChar function with the
        // example string and value of k
        char result = kthNonRepeatingChar(str, k);
 
        // Check if the result variable contains a non-null
        // character and print the appropriate message
        if (result == '\0') {
            System.out.println(
                "There is no kth non-repeating character "
                + "in the string.");
        }
        else {
            System.out.println(
                "The " + k
                + "th non-repeating character in the string "
                + "is " + result + ".");
        }
    }
}

                    

Python3

# Define a function to find the kth non-repeating character
# in a string
def kthNonRepeatingChar(s: str, k: int) -> str:
    # Initialize count and result variables to 0 and null
    # character, respectively
    count = 0
    result = '\0'
 
    # Loop through each character in the string
    for i in range(len(s)):
        # Assume that the current character does not repeat
        repeating = False
 
        # Loop through the rest of the string to check if
        # the current character repeats
        for j in range(i + 1, len(s)):
            if s[i] == s[j]:
                # If the current character repeats, set the
                # repeating flag to true and exit the loop
                repeating = True
                break
 
        # If the current character does not repeat,
        # increment the count of non-repeating characters
        if not repeating:
            count += 1
            # If the count of non-repeating characters
            # equals k, set the result variable to the
            # current character and exit the loop
            if count == k:
                result = s[i]
                break
 
    # Return the result variable
    return result
 
# Define the main function to test the kthNonRepeatingChar
# function
if __name__ == '__main__':
    # Define an example string and value of k
    s = "geeksforgeeks"
    k = 3
 
    # Call the kthNonRepeatingChar function with the
    # example string and value of k
    result = kthNonRepeatingChar(s, k)
 
    # Check if the result variable contains a non-null
    # character and print the appropriate message
    if result == '\0':
        print("There is no kth non-repeating character "
                "in the string.")
    else:
        print(f"The {k}th non-repeating character in the string "
                f"is {result}.")
 
# This code is contributed by Susobhan Akhuli

                    

C#

using System;
 
public class Program {
    // Define a function to find the kth non-repeating
    // character in a string
    static char kthNonRepeatingChar(string str, int k)
    {
        // Initialize count and result variables to 0 and
        // null character, respectively
        int count = 0;
        char result = '\0';
 
        // Loop through each character in the string
        for (int i = 0; i < str.Length; i++) {
            // Assume that the current character does not
            // repeat
            bool repeating = false;
 
            // Loop through the rest of the string to check
            // if the current character repeats
            for (int j = i + 1; j < str.Length; j++) {
                if (str[i] == str[j]) {
                    // If the current character repeats, set
                    // the repeating flag to true and exit
                    // the loop
                    repeating = true;
                    break;
                }
            }
 
            // If the current character does not repeat,
            // increment the count of non-repeating
            // characters
            if (!repeating) {
                count++;
                // If the count of non-repeating characters
                // equals k, set the result variable to the
                // current character and exit the loop
                if (count == k) {
                    result = str[i];
                    break;
                }
            }
        }
 
        // Return the result variable
        return result;
    }
 
    // Define the main function to test the
    // kthNonRepeatingChar function
    static void Main(string[] args)
    {
        // Define an example string and value of k
        string str = "geeksforgeeks";
        int k = 3;
 
        // Call the kthNonRepeatingChar function with the
        // example string and value of k
        char result = kthNonRepeatingChar(str, k);
 
        // Check if the result variable contains a non-null
        // character and print the appropriate message
        if (result == '\0') {
            Console.WriteLine(
                "There is no kth non-repeating character "
                + "in the string.");
        }
        else {
            Console.WriteLine(
                "The " + k
                + "th non-repeating character in the string is "
                + result + ".");
        }
    }
}
 
// This code is contributed by Susobhan Akhuli

                    

Javascript

// Define a function to find the kth non-repeating character
// in a string
function kthNonRepeatingChar(str, k) {
  // Initialize count and result variables to 0 and null
  // character, respectively
  let count = 0;
  let result = "\0";
 
  // Loop through each character in the string
  for (let i = 0; i < str.length; i++) {
    // Assume that the current character does not repeat
    let repeating = false;
 
    // Loop through the rest of the string to check if
    // the current character repeats
    for (let j = i + 1; j < str.length; j++) {
      if (str[i] == str[j]) {
        // If the current character repeats, set the
        // repeating flag to true and exit the loop
        repeating = true;
        break;
      }
    }
 
    // If the current character does not repeat,
    // increment the count of non-repeating characters
    if (!repeating) {
      count++;
      // If the count of non-repeating characters
      // equals k, set the result variable to the
      // current character and exit the loop
      if (count == k) {
        result = str[i];
        break;
      }
    }
  }
 
  // Return the result variable
  return result;
}
 
// Define the main function to test the kthNonRepeatingChar
// function
 
// Define an example string and value of k
let str = "geeksforgeeks";
let k = 3;
 
// Call the kthNonRepeatingChar function with the
// example string and value of k
let result = kthNonRepeatingChar(str, k);
 
// Check if the result variable contains a non-null
// character and print the appropriate message
if (result == "\0") {
  console.log("There is no kth non-repeating character in the string.");
} else {
  console.log(`The ${k}th non-repeating character in the string is ${result}`);
}

                    

Output
The 3th non-repeating character in the string is r.

Time Complexity: O(n²)
Auxiliary Space: O(1)

K’th Non-repeating Character using Hashing:

  • Create an empty hash map to store character counts.
  • Loop through the string and update the counts of each character in the hash map.
  • Loop through the string again and find the kth non-repeating character by checking the count of each character in the hash map.

Below is the implementation of the above idea:

C++

#include <bits/stdc++.h>
using namespace std;
 
char kthNonRepeatingChar(string str, int k)
{
    // Create an empty hash map to store the counts of each
    // character in the string
    unordered_map<char, int> charCounts;
 
    // Loop through the string and store the counts of each
    // character in the hash map
    for (int i = 0; i < str.length(); i++) {
        charCounts[str[i]]++;
    }
 
    // Loop through the string and find the kth
    // non-repeating character
    int nonRepeatingCount = 0;
    for (int i = 0; i < str.length(); i++) {
        if (charCounts[str[i]] == 1) {
            nonRepeatingCount++;
            if (nonRepeatingCount == k) {
               
                // When the count of non-repeating
                // characters equals k, return the character
                return str[i];
            }
        }
    }
 
    // If there is no kth non-repeating character in the
    // string, return the null character
    return '\0';
}
 
int main()
{
    string str = "geeksforgeeks";
    int k = 3;
 
    char result = kthNonRepeatingChar(str, k);
 
    if (result == '\0') {
        cout << "There is no kth non-repeating character "
                "in the string.\n";
    }
    else {
        cout << "The " << k
             << "th non-repeating character in the string "
                "is "
             << result << ".\n";
    }
 
    return 0;
}

                    

Java

import java.util.*;
 
public class Main {
    public static char kthNonRepeatingChar(String str, int k) {
        // Create an empty hash map to store the counts of each
        // character in the string
        Map<Character, Integer> charCounts = new HashMap<>();
 
        // Loop through the string and store the counts of each
        // character in the hash map
        for (int i = 0; i < str.length(); i++) {
            char c = str.charAt(i);
            charCounts.put(c, charCounts.getOrDefault(c, 0) + 1);
        }
 
        // Loop through the string and find the kth
        // non-repeating character
        int nonRepeatingCount = 0;
        for (int i = 0; i < str.length(); i++) {
            char c = str.charAt(i);
            if (charCounts.get(c) == 1) {
                nonRepeatingCount++;
                if (nonRepeatingCount == k) {
                    // When the count of non-repeating
                    // characters equals k, return the character
                    return c;
                }
            }
        }
 
        // If there is no kth non-repeating character in the
        // string, return the null character
        return '\0';
    }
 
    public static void main(String[] args) {
        String str = "geeksforgeeks";
        int k = 3;
 
        char result = kthNonRepeatingChar(str, k);
 
        if (result == '\0') {
            System.out.println("There is no kth non-repeating character " +
                "in the string.");
        } else {
            System.out.println("The " + k + "th non-repeating character " +
                "in the string is " + result + ".");
        }
    }
}

                    

Python3

# Python code of the above approach
def kth_non_repeating_char(string, k):
    # Create an empty dictionary to store
    # the counts of each character in the string
    char_counts = {}
 
    # Loop through the string and store
    # the counts of each character in the dictionary
    for char in string:
        if char in char_counts:
            char_counts[char] += 1
        else:
            char_counts[char] = 1
 
    # Loop through the string and find the kth non-repeating character
    non_repeating_count = 0
    for char in string:
        if char_counts[char] == 1:
            non_repeating_count += 1
            if non_repeating_count == k:
                # When the count of non-repeating
                # characters equals k, return the character
                return char
 
    # If there is no kth non-repeating character in the string, return None
    return None
 
# Main function
if __name__ == "__main__":
    string = "geeksforgeeks"
    k = 3
 
    result = kth_non_repeating_char(string, k)
 
    if result is None:
        print("There is no kth non-repeating character in the string.")
    else:
        print(f"The {k}th non-repeating character in the string is {result}.")
 
# This code is contributed by Susobhan Akhuli

                    

C#

using System;
using System.Collections.Generic;
 
class Gfg
{
    static char kthNonRepeatingChar(string str, int k)
    {
        // Create a dictionary to store the counts of each character in the string
        Dictionary<char, int> charCounts = new Dictionary<char, int>();
 
        // Loop through the string and store
      // the counts of each character in the dictionary
        foreach (char c in str)
        {
            if (charCounts.ContainsKey(c))
            {
                charCounts++;
            }
            else
            {
                charCounts = 1;
            }
        }
 
        // Loop through the string and find
      // the kth non-repeating character
        int nonRepeatingCount = 0;
        foreach (char c in str)
        {
            if (charCounts == 1)
            {
                nonRepeatingCount++;
                if (nonRepeatingCount == k)
                {
                    // When the count of non-repeating characters equals k, return the character
                    return c;
                }
            }
        }
 
        // If there is no kth non-repeating character in the string, return the null character
        return '\0';
    }
 
    static void Main()
    {
        string str = "geeksforgeeks";
        int k = 3;
 
        char result = kthNonRepeatingChar(str, k);
 
        if (result == '\0')
        {
            Console.WriteLine("There is no kth non-repeating character in the string.");
        }
        else
        {
            Console.WriteLine($"The {k}th non-repeating character in the string is {result}.");
        }
    }
}

                    

Javascript

function kthNonRepeatingChar(str, k) {
    // Create an empty map to store the counts of each
    // character in the string
    const charCounts = new Map();
 
    // Loop through the string and store the counts of each
    // character in the map
    for (let i = 0; i < str.length; i++) {
        const char = str[i];
        charCounts.set(char, (charCounts.get(char) || 0) + 1);
    }
 
    // Loop through the string and find the kth non-repeating character
    let nonRepeatingCount = 0;
    for (let i = 0; i < str.length; i++) {
        const char = str[i];
        if (charCounts.get(char) === 1) {
            nonRepeatingCount++;
            if (nonRepeatingCount === k) {
                // When the count of non-repeating characters equals k,
                // return the character
                return char;
            }
        }
    }
 
    // If there is no kth non-repeating character in the string,
    // return the null character
    return '\0';
}
 
// Main function
function main() {
    const str = "geeksforgeeks";
    const k = 3;
 
    const result = kthNonRepeatingChar(str, k);
 
    if (result === '\0') {
        console.log("There is no kth non-repeating character in the string.");
    } else {
        console.log(`The ${k}th non-repeating character in the string is ${result}.`);
    }
}
 
main();

                    

Output
The 3th non-repeating character in the string is r.

Time Complexity: O(n)
Auxiliary Space: O(1)

This article is contributed by Aarti_Rathi and Shivam Gupta.

 



Last Updated : 29 Sep, 2023
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