There exists an array-based, in-place algorithm for this, reducing the need to allocate trees and chase pointers. - The Lost Huffman Paper / HN / wikipedia

• rANS is now the way to go ? - renormalization-based Asymmetric Numerical Systems) compression is a class of entropy coding algorithms that achieves high compression ratios by encoding data using adaptive probability distributions.

see also

• Asymmetric numeral systems (ANS)
• better than Huffman - intro to arithmetic coding