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binary_search.py

 # 使用二分查找法，找出9和20在列表里面的索引
 # 使用二分查找法，找出9和20在列表里面的索引
 num_list = [1, 3, 5, 8, 11, 15, 17, 18, 20, 21]
 num_list = [1, 3, 5, 8, 11, 15, 17, 18, 20, 21]
-num = 9
-low = 0
-high = len(num_list)-1
-mid = (low+high)//2
-if num_list[mid]==num:
-    print("找到了，它在列表里的索引为："+mid)
-elif num_list[mid]<num:
-    low = mid+1
-else:
-    high = mid-1
+
+def binary_search(alist,num):
+    low = 0
+    high = len(num_list)-1
+    while low<=high:
+        mid = (low+high)//2
+        guess = alist[mid]
+        if guess == num:
+            return mid
+        elif guess < num:
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+            low = mid + 1
+        elif guess > num:
+            high = mid - 1
+    return None
+resalt = binary_search(num_list,20)
+print(resalt)
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