What is the most efficient(*) of building a canonical huffman tree?

If frequencies form a monotonic sequence, ie. A0=A1>=...>=An-1, then you can generate an optimal code lengths in O(n) time and O(1) additional space. This algorithm requires only 2 simple passes over the array and it's very fast.

A full description is given in 1.

As I recall, to build a Huffman tree you start with the two least-common leaves and combine them, which removes them from the set and replaces them with a new item that's more common than either (having a count that's the sum of the other two). Then repeat, until there's only one item.

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