387. First Unique Character in a String

Given a string, find the first non-repeating character in it and return its index. If it doesn't exist, return -1.

Examples:

s = "leetcode"
return 0.

s = "loveleetcode"
return 2.

# @param {String} s
# @return {Integer}
def first_uniq_char(s)
  return -1 if s.chars.empty?
  hash = Hash[s.chars.group_by{|x| x}.map{|k,v|[k,v.size]}]
  arr = hash.select{|k,v| v == 1}
  if arr.empty?
    -1
  else
    s.index(arr.first[0])
  end
end