Add Tarjan's algorithm.

src/algorithms/graph/eulerian-path/README.md

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+# Eulerian Path
+
+In graph theory, an **Eulerian trail** (or **Eulerian path**) is a 
+trail in a finite graph which visits every edge exactly once.
+Similarly, an **Eulerian circuit** or **Eulerian cycle** is an 
+Eulerian trail which starts and ends on the same vertex.
+
+Euler proved that a necessary condition for the existence of Eulerian 
+circuits is that all vertices in the graph have an even degree, and 
+stated that connected graphs with all vertices of even degree have 
+an Eulerian circuit.
+
+![Eulerian Circuit](https://upload.wikimedia.org/wikipedia/commons/7/72/Labelled_Eulergraph.svg)
+
+Every vertex of this graph has an even degree. Therefore, this is 
+an Eulerian graph. Following the edges in alphabetical order gives 
+an Eulerian circuit/cycle.
+
+For the existence of Eulerian trails it is necessary that zero or 
+two vertices have an odd degree; this means the Königsberg graph 
+is not Eulerian. If there are no vertices of odd degree, 
+all Eulerian trails are circuits. If there are exactly two vertices 
+of odd degree, all Eulerian trails start at one of them and end at 
+the other. A graph that has an Eulerian trail but not an Eulerian 
+circuit is called semi-Eulerian.
+
+![Königsberg graph](https://upload.wikimedia.org/wikipedia/commons/9/96/K%C3%B6nigsberg_graph.svg)
+
+The Königsberg Bridges multigraph. This multigraph is not Eulerian, 
+therefore, a solution does not exist.
+
+## References
+
+- [Wikipedia](https://en.wikipedia.org/wiki/Eulerian_path)