adding the robin hood code

lectures/optimization/robin_hood/README.md

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+# `robin_hood::unordered_map` — Fast Hash Map for C++
+
+`robin_hood::unordered_map` is a high-performance hash table implementation designed as a faster alternative to `std::unordered_map`, with better cache locality and reduced memory overhead.
+
+---
+
+## Key Features
+
+- **Faster lookups** and insertions compared to `std::unordered_map`
+- **Robin Hood hashing**: minimizes probe length differences to keep performance stable
+- **Header-only**: easy to integrate
+- **Memory efficient**: lower memory usage due to compact internal representation
+- **Supports custom hash and equality functions**
+
+---
+
+## Installation
+
+Include the header file in your project:
+
+```cpp
+#include "robin_hood.h"
+```
+
+You can get the header from:
+
+GitHub: https://github.com/martinus/robin-hood-hashing
+
+## Benchmarks
+
+Compile and run this [program](robin_hood_test.cpp) to see the performance difference; you will need to both [robin_hood_test.cpp](robin_hood_test.cpp) and [robin_hood.h](robin_hood.h).
+
+The program performs 2 millions times of insert and lookup on an std::unordered_map container; and then performs 2 million times of insert and lookup on a robin_hood::unordered_map container. As can be seen, the robin hood test runs much faster.
+
+```console
+$g++ -o robin_hood_test robin_hood_test.cpp
+$./robin_hood_test
+[std::unordered_map] Time: 1145 ms. Sum: -1455759936
+[robin_hood::unordered_map] Time: 918 ms. Sum: -1455759936
+```