Add base paper and tarjan's SCC algorithm

graph/vertex.h

@@ -0,0 +1,36 @@
+#ifndef VERTEX_H_
+#define VERTEX_H_
+
+#include <cstdint>
+#include <compare>
+#include <ostream>
+#include <vector>
+
+namespace graph {
+
+template<typename T>
+class Vertex {
+public:
+  Vertex() = default;
+  Vertex(T v) : v(v) {}
+  
+  T v;
+  std::uint16_t index = -1;
+  std::uint16_t lowlink = -1;
+  bool onStack = false;
+  std::vector<Vertex<T>> edges;
+
+  auto operator<=>(const Vertex&) const = default;
+
+  template<typename T>
+  friend inline std::ostream& operator<<(std::ostream& os, const Vertex<T>& v);
+};
+
+template<typename T>
+inline std::ostream& operator<<(std::ostream& os, const Vertex<T>& vout) {
+  return os << vout.v;
+}
+
+} // namespace graph
+
+#endif