Python - Program to Find the First Repeating Character in a String

def first_repeating_char(s):

    seen = set()  # A set to keep track of characters we've seen

    for char in s:

        if char in seen:

            return char  # Return the first repeating character

        seen.add(char)

    return None  # Return None if no repeating character is found

# Example Usage

string = "abca"

result = first_repeating_char(string)

if result:

    print(f"The first repeating character is: {result}")

else:

    print("No repeating characters found.")

Explanation:

Initialize a Set (seen):

This keeps track of characters that have already been encountered.

Iterate Through the String (for char in s):

If the character is already in the seen set, it means we've encountered it before → Return that character as the first repeating character.

If the character is not in seen, add it to the set.

If No Repeating Character is Found:

Return None.

Example Run:

string = "abca"

Iteration 1: 'a' → add to seen

Iteration 2: 'b' → add to seen

Iteration 3: 'c' → add to seen

Iteration 4: 'a' → already in seen, so return 'a'

Output:

The first repeating character is: a

This solution runs in O(n) time complexity, making it efficient for large strings.