number-of-digit-one.rb

# @param {Integer} n
# @return {Integer}
def count_digit_one(n)
  digit = 10
  sum = 0
  while n > 0
    base = 10 ** digit
    msb = n / base
    lsb = n % base
    # 3[XXXX] (0-29999)
    sum += msb * digit * base / 10 if n >= base
    # [3]XXXX (10000-19999)
    sum += base                    if msb >= 2
    # [1]XXXX (10000-1XXXX)
    sum += lsb + 1                 if msb == 1
    n = lsb
    digit -= 1
  end
  sum
end