add Strategy pattern

strategy/README.txt

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+## Strategy
+
+Strategy defines a family of algorithms, encapsulates each one, and makes them 
+interchangeable. It lets the algorithm vary independently fromclients that use it. 
+The pattern has behavioral purpose and applies to the objects.
+
+### When to use
+
+* many related classes differ only in their behavior
+* you need different variants of an algorithm
+* an algorithm uses data that clients shouldn't know about

strategy/Strategy.cpp

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+/*
+ * C++ Design Patterns: Strategy
+ * Author: Jakub Vojvoda [github.com/JakubVojvoda]
+ * 2016
+ *
+ * Source code is licensed under MIT License
+ * (for more details see LICENSE)
+ *
+ */
+
+#include <iostream>
+
+/*
+ * Strategy
+ * declares an interface common to all supported algorithms
+ */
+class Strategy {
+public:
+  virtual ~Strategy() { /* ... */ }
+  virtual void algorithmInterface() = 0;
+  // ...
+};
+
+/*
+ * Concrete Strategies
+ * implement the algorithm using the Strategy interface
+ */
+class ConcreteStrategyA : public Strategy {
+public:
+  ~ConcreteStrategyA() { /* ... */ }
+
+  void algorithmInterface() {
+    std::cout << "Concrete Strategy A" << std::endl;
+  }
+  // ...
+};
+
+class ConcreteStrategyB : public Strategy {
+public:
+  ~ConcreteStrategyB() { /* ... */ }
+
+  void algorithmInterface() {
+    std::cout << "Concrete Strategy B" << std::endl;
+  }
+  // ...
+};
+
+class ConcreteStrategyC : public Strategy {
+public:
+  ~ConcreteStrategyC() { /* ... */ }
+
+  void algorithmInterface() {
+    std::cout << "Concrete Strategy C" << std::endl;
+  }
+  // ...
+};
+
+/*
+ * Context
+ * maintains a reference to a Strategy object
+ */
+class Context {
+public:
+  Context(Strategy *s)
+    : strategy(s) {}
+
+  ~Context() {
+    delete strategy;
+  }
+
+  void contextInterface() {
+    strategy->algorithmInterface();
+  }
+  // ...
+
+private:
+  Strategy *strategy;
+  // ...
+};
+
+
+int main()
+{
+  ConcreteStrategyA strategy;
+  // ConcreteStrategyB strategy;
+  // ConcreteStrategyC strategy;
+
+  Context context(&strategy);
+  context.contextInterface();
+
+  return 0;
+}