add python code

public/py/ch4/huffman/huffman.py

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+"""
+Huffman 编码 — 贪心算法
+构建最优前缀码实现数据压缩
+"""
+import heapq
+from collections import Counter
+
+class HuffmanNode:
+    def __init__(self, char, freq):
+        self.char = char
+        self.freq = freq
+        self.left = None
+        self.right = None
+
+    def __lt__(self, other):
+        return self.freq < other.freq
+
+def build_huffman_tree(text):
+    """构建 Huffman 树"""
+    freq = Counter(text)
+    heap = [HuffmanNode(char, f) for char, f in freq.items()]
+    heapq.heapify(heap)
+
+    while len(heap) > 1:
+        left = heapq.heappop(heap)
+        right = heapq.heappop(heap)
+        merged = HuffmanNode(None, left.freq + right.freq)
+        merged.left = left
+        merged.right = right
+        heapq.heappush(heap, merged)
+
+    return heap[0] if heap else None
+
+def generate_codes(node, prefix="", code_map=None):
+    """从 Huffman 树生成编码"""
+    if code_map is None:
+        code_map = {}
+
+    if node is None:
+        return code_map
+
+    if node.char is not None:
+        code_map[node.char] = prefix or "0"
+    else:
+        generate_codes(node.left, prefix + "0", code_map)
+        generate_codes(node.right, prefix + "1", code_map)
+
+    return code_map
+
+if __name__ == "__main__":
+    text = "ABRACADABRA"
+    print(f"原文: {text}")
+
+    root = build_huffman_tree(text)
+    codes = generate_codes(root)
+
+    print("\nHuffman 编码:")
+    for char, code in sorted(codes.items()):
+        print(f"  '{char}': {code}")
+
+    encoded = "".join(codes[c] for c in text)
+    print(f"\n编码结果: {encoded}")
+    print(f"原始大小: {len(text) * 8} bits")
+    print(f"压缩大小: {len(encoded)} bits")

public/py/ch4/huodong/activity_selection.py

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+"""
+活动选择问题 — 贪心算法
+选择最多的不重叠活动
+"""
+
+def activity_selection(start, finish):
+    """贪心选择最早结束的活动"""
+    n = len(start)
+    # 按结束时间排序
+    activities = sorted(zip(start, finish), key=lambda x: x[1])
+
+    selected = [activities[0]]
+    last_finish = activities[0][1]
+
+    for i in range(1, n):
+        if activities[i][0] >= last_finish:
+            selected.append(activities[i])
+            last_finish = activities[i][1]
+
+    return selected
+
+if __name__ == "__main__":
+    start = [1, 3, 0, 5, 8, 5]
+    finish = [2, 4, 6, 7, 9, 9]
+
+    result = activity_selection(start, finish)
+    print("选择的活动 (开始, 结束):")
+    for s, f in result:
+        print(f"  [{s}, {f})")
+    print(f"共 {len(result)} 个活动")

public/py/ch4/money/coin_change.py

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+"""
+找零问题 — 贪心算法
+使用最少硬币凑出指定金额
+"""
+
+def min_coins_greedy(coins, amount):
+    """贪心找零（硬币面额需满足贪心选择性质）"""
+    coins_sorted = sorted(coins, reverse=True)
+    result = []
+    remaining = amount
+
+    for coin in coins_sorted:
+        while remaining >= coin:
+            result.append(coin)
+            remaining -= coin
+
+    return result
+
+def min_coins_dp(coins, amount):
+    """动态规划找零（通用解法）"""
+    MAX = float('inf')
+    dp = [MAX] * (amount + 1)
+    dp[0] = 0
+
+    for i in range(1, amount + 1):
+        for coin in coins:
+            if i >= coin:
+                dp[i] = min(dp[i], dp[i - coin] + 1)
+
+    return dp[amount] if dp[amount] != MAX else -1
+
+if __name__ == "__main__":
+    coins = [1, 5, 10, 25]
+    amount = 63
+
+    greedy_result = min_coins_greedy(coins, amount)
+    dp_result = min_coins_dp(coins, amount)
+
+    print(f"硬币面额: {coins}")
+    print(f"需要凑出: {amount}")
+    print(f"贪心结果: {greedy_result} (共 {len(greedy_result)} 枚)")
+    print(f"DP最少数量: {dp_result} 枚")