Quick Sort (In-Place) - Ruby

#! /usr/bin/env ruby

# str is array of chars
# returns type like `str`
def quicksort(str, _begin, _end)
    return str[_begin..._end] if _end - _begin <= 1

    pivot = str[_end - 1] # choose last char as pivot
    a = _begin
    b = _end - 1

    while (b > a)
        if str[b-1] > pivot
            str[b] = str[b-1]
            b-=1
        else
            str[b-1],str[a] = str[a],str[b-1]
            a+=1
        end
    end

    str[b] = pivot

    str[ _begin...b ] = quicksort(str, _begin, b) if b - _begin > 1
    str[ (b+1)..._end] = quicksort(str, b+1, _end) if _end - b > 1

    return str[_begin..._end]

end

puts "give me a string"
str = gets.chomp.split('')

puts quicksort(str, 0, str.length).join('')