Add Postman Sort (LSD Radix Sort) in R

[Siddhram]

This implementation provides decimal LSD (Least Significant Digit) Radix Sort in R.
It sorts non-negative integers by repeatedly applying a stable counting sort to each digit, from the least significant to the most significant.

Overview

• Algorithm Type: Non-comparative, stable, digit-based sorting
• Target Data: Non-negative integer vectors
• Execution: Iteratively sorts by each digit place (1s, 10s, 100s, ...) using counting sort
• Notes: Mirrors typical Java implementations; uses base-10 digit processing

Features

• Stable sorting per digit ensures the relative order of equal elements is preserved
• Efficient for integers with limited digit width
• Easy to understand and implement using counting sort
• Handles large numbers by increasing the exponent (exp) iteratively

Complexity

• Time Complexity (TC): O(d × (n + k))
  • n = number of elements, d = number of digits in the maximum element, k = radix/base (10 here)
• Space Complexity (SC): O(n + k)
  • Temporary array for output + counting array per digit