JavaScript Program to Print All Duplicate Characters in a String

In this article, we will learn how to print all duplicate characters in a string in JavaScript. Given a string S, the task is to print all the duplicate characters with their occurrences in the given string.

Example:

Input: S = “geeksforgeeks”
Output:
e, count = 4
g, count = 2
k, count = 2
s, count = 2

Examples of Print All Duplicate Characters in a String In JavaScript

1. Using For Loop in JavaScript

In this approach, an object named charCount to effectively maintain the frequency of characters in the string. It sequentially processes each character in the input string, increasing its count in the charCount object. Once the character frequencies are established, the code then traverses through the charCount object. It selectively prints characters that have occurrences greater than one, thereby accurately showcasing the duplicate characters along with the corresponding count of duplications.

Example: In this example, we are using a for loop.

Javascript

function printDups(str) {
    let charCount = {};

    for (let i = 0; i < str.length; i++) {
        let character = str[i];
        charCount[character] =
            (charCount[character] || 0) + 1;
    }

    for (let char in charCount) {
        if (charCount[char] > 1) {
            console.log(
                char +
                    ", count = " +
                    charCount[char]
            );
        }
    }
}

let str = "geeksforgeeks";
printDups(str);

Output

g, count = 2
e, count = 4
k, count = 2
s, count = 2

2.Using Sorting in JavaScript

• Sort the given string.
• Loop through the sorted string to find the duplicates.
• If the next character is the same as the current character then we keep on counting the occurrence of that char.
• If the count is greater than one then we print the character and its count.

Example: In this example we are using Sorting

Javascript

function printDuplicates(str) {
    let len = str.length;

    // Sorting the string
    str = str.split('').sort().join('');

    // Loop through the sorted string to find duplicates
    for (let i = 0; i < len; i++) {
        let count = 1;

        // Counting the occurrences of each character
        while (i < len - 1 && str[i] == str[i + 1]) {
            count++;
            i++;
        }

        // Printing the duplicate character and its count
        if (count > 1) {
            console.log(str[i] + ", count = " + count);
        }
    }
}

let str = "geeksforgeeks";
printDuplicates(str);

Output

e, count = 4
g, count = 2
k, count = 2
s, count = 2

3.Using Hashing in JavaScript

• Declare a unordered map of the char-int pair.
• Traverse the string using a loop and increase the count of the present char in the map.
• Iterate through the map and print a character that has a value greater than one in map.

Example: In this example we are using Hashing.

Javascript

// JavaScript program to count all duplicates
// from string using maps
function printDups(str) {
    let count = new Map();
    for (let i = 0; i < str.length; i++) {
        if (count.has(str[i])) {
            count.set(
                str[i],
                count.get(str[i]) + 1
            );
        } else {
            count.set(str[i], 1);
        }
        //increase the count of characters by 1
    }
    //iterating through the unordered map
    for (let [it, it2] of count) {
        if (it2 > 1)
            //if the count of characters is
            //greater than 1 then duplicate found
            console.log(it, ", count = ", it2);
    }
}
/* Driver code*/

let str = "geeksforgeeks";
printDups(str);

Output

g , count =  2
e , count =  4
k , count =  2
s , count =  2

4.Using Set()

This approach uses two Sets to track characters in a string, seen set to store characters encountered, and duplicates set to store duplicate characters. It iterates over the string, adding characters to seen if they are not already there, or to duplicates if they are.

Example: In this example we are using Set().

JavaScript

function findDuplicates(str) {
  const seen = new Set();
  const duplicates = new Set();

  for (let char of str) {
    if (seen.has(char)) {
      duplicates.add(char);
    } else {
      seen.add(char);
    }
  }

  duplicates.forEach(char => {
    const count = str.split(char).length - 1;
    console.log(`${char}, count= ${count}`);
  });
}

const str = "geeksforgeeks";
findDuplicates(str);

Output

e, count= 4
g, count= 2
k, count= 2
s, count= 2