first_letter_to_appear_twice.py

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

# Note:

# A letter a appears twice before another letter b if the second occurrence of a is before the second occurrence of b.
# s will contain at least one letter that appears twice.
 

# Example 1:

# Input: s = "abccbaacz"
# Output: "c"
# Explanation:
# The letter 'a' appears on the indexes 0, 5 and 6.
# The letter 'b' appears on the indexes 1 and 4.
# The letter 'c' appears on the indexes 2, 3 and 7.
# The letter 'z' appears on the index 8.
# The letter 'c' is the first letter to appear twice, because out of all the letters the index of its second occurrence is the smallest.
# Example 2:

# Input: s = "abcdd"
# Output: "d"
# Explanation:
# The only letter that appears twice is 'd' so we return 'd'.

class Solution:
    def repeatedCharacter(self, s: str) -> str:
        tracker = {}
        
        for letter in s:
            if letter not in tracker:
                tracker[letter] = 1
            else:
                return letter