C++11 solution to the 8th chapter problem. (#145)

* C++11 solution to 8th chapter problem.

* Update 01_set_covering.cpp

* Extra space correction

08_greedy_algorithms/c++11/01_set_covering.cpp

@@ -0,0 +1,59 @@
+#include <iostream>
+#include <unordered_set>
+#include <unordered_map>
+
+std::unordered_set<std::string> operator & (const std::unordered_set<std::string>&, 
+				            const std::unordered_set<std::string>&);
+void operator -= (std::unordered_set<std::string>&, 
+	          const std::unordered_set<std::string>&);
+
+int main() {
+	std::unordered_set<std::string> statesNeeded({ "mt", "wa", "or", "id", "nv", "ut", "ca", "az" });
+
+	std::unordered_map<std::string, std::unordered_set<std::string>> stations;
+	stations.insert({ "kone", {"id", "nv", "ut"} });
+	stations.insert({ "ktwo", {"wa", "id", "mt" } });
+	stations.insert({ "kthree", {"or", "nv", "ca" } });
+	stations.insert({ "kfour", {"nv", "ut" } });
+	stations.insert({ "kfive", {"ca", "az" } });
+
+	std::unordered_set<std::string> finalStations;
+
+	while (!statesNeeded.empty()) {
+		std::string bestStation;
+		std::unordered_set <std::string> statesCovered;
+		for (const auto& i : stations) {
+			std::unordered_set<std::string> coverage = i.second & statesNeeded;
+			if (coverage.size() > statesCovered.size()) {
+				bestStation = i.first;
+				statesCovered = coverage;
+			}
+		}
+		statesNeeded -= statesCovered;
+		finalStations.insert(bestStation);
+	}
+	
+	for (const auto& i : finalStations)
+		std::cout << i << std::endl;
+
+	system("pause");
+	return 0;
+}
+
+ std::unordered_set<std::string> operator & (const std::unordered_set<std::string>& a,
+					     const std::unordered_set<std::string>& b) {
+	 std::unordered_set<std::string> result;
+	 for (const auto& i : a)
+		 for (const auto& j : b)
+			 if (i == j)
+				 result.insert(i);
+	 return result;
+ }
+
+void operator -= (std::unordered_set<std::string>& a,
+		  const std::unordered_set<std::string>& b) {
+	for (auto j = b.begin(); j != b.end(); ++j)
+		for (auto i = a.begin(); i != a.end(); )
+			if (*i == *j) i = a.erase(i);
+			else ++i;
+ }