电文的编码和译码 任务：从键盘接收一串电文字符，输出对应的哈夫曼编码。同时，能翻译由哈夫曼编码生成的代码串，输出对应的电文字符串。 ①构造一棵哈夫曼树；②实现哈夫曼编码，并用哈夫曼编码生成的代码串进行译码；③程序中字符和权值是可变的，实现程序的灵活性。

实现代码如下：

①哈夫曼树的构造：

class Node:
    def __init__(self, value, freq):
        self.value = value
        self.freq = freq
        self.left = None
        self.right = None

def construct_huffman_tree(freq_dict):
    nodes = [Node(value, freq) for value, freq in freq_dict.items()]
    while len(nodes) > 1:
        nodes = sorted(nodes, key=lambda x: x.freq)
        left_node, right_node = nodes.pop(0), nodes.pop(0)
        parent_node = Node(None, left_node.freq + right_node.freq)
        parent_node.left, parent_node.right = left_node, right_node
        nodes.append(parent_node)

    return nodes[0]

其中，freq_dict是一个字典类型，记录每个字符出现的频率。

②哈夫曼编码和译码：

class HuffmanCode:
    def __init__(self, freq_dict):
        self.freq_dict = freq_dict
        self.root = construct_huffman_tree(freq_dict)
        self.codes_dict = self._generate_codes_dict()

    def _generate_codes_dict(self):
        def dfs(node, path):
            if not node:
                return
            if node.value is not None:
                self.codes_dict[node.value] = path
            if node.left:
                dfs(node.left, path + "0")
            if node.right:
                dfs(node.right, path + "1")

        self.codes_dict = {}
        dfs(self.root, "")
        return self.codes_dict

    def encode(self, text):
        result = ""
        for ch in text:
            result += self.codes_dict[ch]
        return result

    def decode(self, code):
        result = ""
        node = self.root
        for bit in code:
            node = node.left if bit == "0" else node.right
            if node.value is not None:
                result += node.value
                node = self.root
        return result

其中，_generate_codes_dict()函数通过遍历哈夫曼树，生成每个字符对应的哈夫曼编码。

③完整代码：

class Node:
    def __init__(self, value, freq):
        self.value = value
        self.freq = freq
        self.left = None
        self.right = None

def construct_huffman_tree(freq_dict):
    nodes = [Node(value, freq) for value, freq in freq_dict.items()]
    while len(nodes) > 1:
        nodes = sorted(nodes, key=lambda x: x.freq)
        left_node, right_node = nodes.pop(0), nodes.pop(0)
        parent_node = Node(None, left_node.freq + right_node.freq)
        parent_node.left, parent_node.right = left_node, right_node
        nodes.append(parent_node)

    return nodes[0]

class HuffmanCode:
    def __init__(self, freq_dict):
        self.freq_dict = freq_dict
        self.root = construct_huffman_tree(freq_dict)
        self.codes_dict = self._generate_codes_dict()

    def _generate_codes_dict(self):
        def dfs(node, path):
            if not node:
                return
            if node.value is not None:
                self.codes_dict[node.value] = path
            if node.left:
                dfs(node.left, path + "0")
            if node.right:
                dfs(node.right, path + "1")

        self.codes_dict = {}
        dfs(self.root, "")
        return self.codes_dict

    def encode(self, text):
        result = ""
        for ch in text:
            result += self.codes_dict[ch]
        return result

    def decode(self, code):
        result = ""
        node = self.root
        for bit in code:
            node = node.left if bit == "0" else node.right
            if node.value is not None:
                result += node.value
                node = self.root
        return result


freq_dict = {}
text = input("请输入电文字符串：")
for ch in text:
    freq_dict[ch] = freq_dict.get(ch, 0) + 1

huffman_code = HuffmanCode(freq_dict)
code = huffman_code.encode(text)
print("哈夫曼编码：", code)

text_decode = huffman_code.decode(code)
print("电文字符串：", text_decode)