require 'set'
# Finds the first character that repeats as you scan a string from left to right.
#
# @param str [String] The input string to evaluate.
# @return [String, nil] The first repeated character, or nil if no characters repeat.
def find_first_repeated_character(str)
seen = Set.new
# Iterate through each character and find the first one where add? returns nil (already exists)
str.each_char.find { |char| !seen.add?(char) }
end
# --- Test Cases ---
test_cases = [
"abca",
"abcbd",
"abcde",
"abcdxypcbop"
]
test_cases.each do |text|
result = find_first_repeated_character(text)
display_result = result || "None"
puts "Input: \"#{text}\" -> First Repeated: #{display_result}"
end
# run:
#
# Input: "abca" -> First Repeated: a
# Input: "abcbd" -> First Repeated: b
# Input: "abcde" -> First Repeated: None
# Input: "abcdxypcbop" -> First Repeated: c
#
