Pseudo Code

Lesson 4 of 9 in Coddy's Radix Sort - DSA Series course.

countingSortByDigit(array, exp):
   n = length(array)
   output = array of size n
   count = array of 10 zeros
   for each x in array:                 # tally each digit
      count[(x / exp) % 10] += 1
   for d from 1 to 9:                   # running totals -> positions
      count[d] += count[d - 1]
   for k from n-1 down to 0:            # place from the back (stable)
      d = (array[k] / exp) % 10
      count[d] -= 1
      output[count[d]] = array[k]
   return output

radixSort(array):
   max = largest value in array
   exp = 1
   while max / exp > 0:
      array = countingSortByDigit(array, exp)
      exp = exp * 10

exp is the place value: 1 for ones, 10 for tens, 100 for hundreds. (x / exp) % 10 pulls out that digit.
countingSortByDigit is a stable sort on a single digit: it counts how many of each digit appear, turns those counts into positions, then places elements from back to front to preserve order.
radixSort just runs that pass once per digit, stopping when exp passes the largest number.

Try it yourself

This lesson doesn't include a code challenge.