Add tarjan algorithm

Signed-off-by: Bruno Freitas Tissei <bft15@inf.ufpr.br>

graph/tarjan.cpp

+/**
+ * Tarjan - Strongly Connected Components
+ *
+ * Complexity (Time): O(n + m)
+ * Complexity (Space): O(n + m)
+ */
+
+vector<int> graph[MAX];
+vector<int> scc[MAX];
+
+stack<int> S;
+int ncomp, ind;
+int cont[MAX], parent[MAX];
+int low[MAX], L[MAX];
+
+// Fills scc with strongly connected components of graph
+void dfs(int x) {
+  L[x] = ind;
+  low[x] = ind++;
+  cont[x] = 1;
+  S.push(x);
+
+  for (auto i : graph[x])
+    if (L[i] == -1) {
+      dfs(i);
+      low[x] = min(low[x], low[i]);
+    } else if (cont[i])
+      low[x] = min(low[x], L[i]);
+
+  if (low[x] == L[x]) {
+    int w;
+    ncomp++;
+
+    do {
+      w = S.top(); S.pop();
+      cont[w] = 0;
+      scc[ncomp].pb(w);
+    } while (w != x);
+  }
+}
+
+
+// Return number of SCCs
+int tarjan(int n) {
+  memset(L, -1, sizeof L);
+  ncomp = ind = 0;
+
+  for (int i = 0; i < n; ++i)
+    if (L[i] == -1)
+      dfs(i);
+
+  return ncomp;
+}