Update Binary_Search.mdx

content/4_Silver/Binary_Search.mdx

@@ -111,6 +111,8 @@ static boolean check(long x){
 }
 ```
 
+Note that I used the `Arrays.sort()` function, which uses quicksort on primitive data types such as `long`s. This is fine for USACO, but in other contests, such as CodeForces, it may time out on test cases specifically engineered to trigger worst-case O(n^2) behavior in quicksort. In such contests, you should declare the underlying array as an array of objects, for example `Long` instead of `long`. This forces the `Arrays.sort()` function to use mergesort, which is always O(n log n).
+
 C++:
 
 ```cpp