帮我写一段代码 输入一串字符串，根据给定的字符串中字符出现的频率建立相应哈夫曼树，构造哈夫曼编码表，在此基础上可以对待压缩文件进行压缩（即编码），同时可以对压缩后的二进制编码文件进行解压（即译码）。

好的，以下是参考代码：

import heapq
from collections import Counter, namedtuple

class HuffmanNode(namedtuple("HuffmanNode", ["left", "right"])):
    def walk(self, code, acc):
        self.left.walk(code, acc + "0")
        self.right.walk(code, acc + "1")

class HuffmanLeaf(namedtuple("HuffmanLeaf", ["char"])):
    def walk(self, code, acc):
        code[self.char] = acc or "0"

def huffman_encode(s):
    heap = []
    for char, freq in Counter(s).items():
        heap.append((freq, len(heap), HuffmanLeaf(char)))
    heapq.heapify(heap)
    count = len(heap)
    while len(heap) > 1:
        freq1, _count1, left = heapq.heappop(heap)
        freq2, _count2, right = heapq.heappop(heap)
        heapq.heappush(heap, (freq1 + freq2, count, HuffmanNode(left, right)))
        count += 1
    code = {}
    if heap:
        [(_freq, _count, root)] = heap
        root.walk(code, "")
    return code

def huffman_compress(s, code):
    return "".join(code[char] for char in s)

def huffman_decompress(c, code):
    inv_code = {v: k for k, v in code.items()}
    chars = []
    bitstring = ""
    for bit in c:
        bitstring += bit
        if bitstring in inv_code:
            chars.append(inv_code[bitstring])
            bitstring = ""
    return "".join(chars)

# 测试
original = "this is a test string"
code = huffman_encode(original)
compressed = huffman_compress(original, code)
print(compressed)
decompressed = huffman_decompress(compressed, code)
print(decompressed)
assert original == decompressed