FIND THE FIRST
REPEATED CHARACTER
IN A GIVEN STRING

When I interview junior developers, I love throwing small-but-smart problems at them. Not because I want to see them sweat but because these little puzzles tell me how someone thinks.

One of my favorites: “Find the first repeated character inside a string.”

Most people overthink it.
Some start building maps.
Others loop twice.
A few bring out regex like they’re going to war.

Let me walk you through it like I would with a teammate.

The Thought Process

The job is simple: We scan characters one by one and return the first character that has appeared before.

So naturally, we need something that:

• check_circle Stores things we’ve seen
• check_circle Lets us check quickly whether something is already there
• check_circle Doesn’t allow duplicates

Sounds familiar? That’s exactly what a Set is made for.

The Clean Java Solution

Here’s the simplest version using only a Set

public Character firstRepeatedChar(String input) {
    if (input == null || input.isEmpty()) return null;

    Set<Character> seen = new HashSet<>();

    for (char c : input.toCharArray()) {
      if (!seen.add(c)) {
          // add() returns false when element already exists
          return c;
      }
    }
    return null; // no repeated character
}

Why I like this:

• check_circle No extra complexity
• check_circle add() returning false becomes your “ding ding!” signal
• check_circle Runs in O(n) time
• check_circle Easy to explain in code review or interviews

Example

Input: "abcaadef"

Output: 'a'