find_the_first_repeated_character_in_a_string.md

Find the First Repeated Character in a String

Find the First Repeated Character in a String

To find the first repeated character in a string, use a set to track characters that have been seen.

Algorithm:

1. Initialize an empty set to store characters.
2. Iterate through the string, checking if each character has already been seen in the set.
3. If a character is found in the set, return it as the first repeated character.
4. If no repeated character is found, return null.

Example:

function findFirstRepeatedCharacter(str) {
    const seen = new Set();
    for (let char of str) {
        if (seen.has(char)) return char;
        seen.add(char);
    }
    return null;
}

// Example usage
console.log(findFirstRepeatedCharacter('abcdeab')); // Output: 'a'
console.log(findFirstRepeatedCharacter('abcdef')); // Output: null

This method has a time complexity of O(n), where n is the length of the string.

Tags: intermediate, JavaScript, Strings, Algorithm