香农，费诺和赫夫曼编码代码

以下是Python实现的三种编码算法：香农编码、费诺编码和赫夫曼编码。

## 香农编码

def shannon_fano(p):
    if len(p) == 1:
        return {p.keys()[0]: '0'}
    s = sum(p.values())
    m = s / 2
    left, right = {}, {}
    for k, v in p.items():
        if m >= s:
            left[k] = v
        else:
            right[k] = v
        s -= v
    code = shannon_fano(left)
    code.update(shannon_fano(right))
    for k in left:
        code[k] = '0' + code[k]
    for k in right:
        code[k] = '1' + code[k]
    return code

## 费诺编码

def huffman(p):
    heap = [[w, [s, '']] for s, w in p.items()]
    heapq.heapify(heap)
    while len(heap) > 1:
        lo = heapq.heappop(heap)
        hi = heapq.heappop(heap)
        for pair in lo[1:]:
            pair[1] = '0' + pair[1]
        for pair in hi[1:]:
            pair[1] = '1' + pair[1]
        heapq.heappush(heap, [lo[0] + hi[0]] + lo[1:] + hi[1:])
    return dict(sorted(heapq.heappop(heap)[1:], key=lambda p: p[0]))

## 赫夫曼编码

def huffman(p):
    heap = [[w, [s, '']] for s, w in p.items()]
    heapq.heapify(heap)
    while len(heap) > 1:
        lo = heapq.heappop(heap)
        hi = heapq.heappop(heap)
        for pair in lo[1:]:
            pair[1] = '0' + pair[1]
        for pair in hi[1:]:
            pair[1] = '1' + pair[1]
        heapq.heappush(heap, [lo[0] + hi[0]] + lo[1:] + hi[1:])
    return dict(sorted(heapq.heappop(heap)[1:], key=lambda p: p[0]))

这些实现都需要一个字典，其中键是字符，值是相应字符的频率。例如，如果要编码字符串“hello world”，则字典应如下所示：

p = {'h': 1, 'e': 1, 'l': 3, 'o': 2, ' ': 1, 'w': 1, 'r': 1, 'd': 1}