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First Letter to Appear Twice

Difficulty: Easy

Problem Description

Given a string s consisting of lowercase English letters, return the first letter to appear twice.

Key Insights

  • We need to identify the first character in the string that appears more than once.
  • The order of appearance matters; the character that reappears first is the desired output.
  • A hash table (or set) can be used to efficiently track characters and their occurrences.

Space and Time Complexity

Time Complexity: O(n), where n is the length of the string s, since we need to traverse the string once.

Space Complexity: O(1), since the size of the hash table (or set) is limited to the number of lowercase English letters (26).

Solution

To solve the problem, we can use a hash table (or set) to keep track of characters we have seen as we iterate through the string. For each character, we check if it has been seen before:

  1. If it has, we immediately return that character as it is the first to appear twice.
  2. If it hasn't, we add it to our hash table and continue.

This approach ensures we only make one pass through the string while keeping track of the characters efficiently.

Code Solutions

def first_letter_to_appear_twice(s: str) -> str:
    seen = set()  # Set to keep track of seen characters
    for char in s:
        if char in seen:  # If we have seen the character before
            return char  # Return it as the first letter that appears twice
        seen.add(char)  # Add the character to the set
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