letcode_1763.py

# A string s is nice if, for every letter of the alphabet that s contains, it appears both in uppercase
#  and lowercase. For example, "abABB" is nice because 'A' and 'a' appear, and 'B' and 'b' appear. However,
#   "abA" is not because 'b' appears, but 'B' does not.

# Given a string s, return the longest substring of s that is nice. If there are multiple, return the 
# substring of the earliest occurrence. If there are none, return an empty string.

 

# Example 1:

# Input: s = "YazaAay"
# Output: "aAa"
# Explanation: "aAa" is a nice string because 'A/a' is the only letter of the alphabet in s, and both 'A' 
# and 'a' appear.
# "aAa" is the longest nice substring.
# Example 2:

# Input: s = "Bb"
# Output: "Bb"
# Explanation: "Bb" is a nice string because both 'B' and 'b' appear. The whole string is a substring.
# Example 3:

# Input: s = "c"
# Output: ""
# Explanation: There are no nice substrings.
# Example 4:

# Input: s = "dDzeE"
# Output: "dD"
# Explanation: Both "dD" and "eE" are the longest nice substrings.
# As there are multiple longest nice substrings, return "dD" since it occurs earlier.