Add a hint

exercises/25/pacman_ai/README.md

@@ -44,6 +44,15 @@ Use the [std::sort](https://en.cppreference.com/w/cpp/algorithm/sort) function t
 
 </details>
 
+</details>
+<details>
+   <summary>Hint 3</summary>
+
+[std::sort](https://en.cppreference.com/w/cpp/algorithm/sort) third parameter should be a lambda taking 2 `GridPosition` as parameter,
+and return true if the first parameter is closer from PacMan than the second.
+
+</details>
+
 ### Part 2
 
 Implement [PacManAI::isValidMove](../../../lib/PacManAI.cpp) and test your implementation with the test
@@ -68,11 +77,11 @@ in [testPacmanAI.cpp](../../../test/testPacmanAI.cpp) called _"Is optimal direct
 ```cpp
 Direction PacManAI::optimalDirection(const std::array<Move, 4> & moves) {
   return Direction::NONE;
-}  
+}
 ```
 
 <details>
    <summary>Hint</summary>
 </details>
 
-   
+