add python code

public/py/ch2/mergesort/mergesort.py

@@ -0,0 +1,50 @@
+"""
+归并排序 — 分治法的经典应用
+时间复杂度 O(n log n)，空间复杂度 O(n)
+"""
+
+def merge(arr, left, mid, right):
+    """合并两个有序子数组"""
+    # 创建临时数组
+    L = arr[left:mid + 1]
+    R = arr[mid + 1:right + 1]
+
+    i = j = 0
+    k = left
+
+    # 合并两个有序数组
+    while i < len(L) and j < len(R):
+        if L[i] <= R[j]:
+            arr[k] = L[i]
+            i += 1
+        else:
+            arr[k] = R[j]
+            j += 1
+        k += 1
+
+    # 复制剩余元素
+    while i < len(L):
+        arr[k] = L[i]
+        i += 1
+        k += 1
+    while j < len(R):
+        arr[k] = R[j]
+        j += 1
+        k += 1
+
+def merge_sort(arr, left, right):
+    """归并排序主函数"""
+    if left < right:
+        mid = (left + right) // 2
+        merge_sort(arr, left, mid)
+        merge_sort(arr, mid + 1, right)
+        merge(arr, left, mid, right)
+
+def sort(arr):
+    merge_sort(arr, 0, len(arr) - 1)
+
+if __name__ == "__main__":
+    data = [38, 27, 43, 3, 9, 82, 10]
+    print("排序前:", data)
+    sort(data)
+    print("排序后:", data)