Add disjoint_set

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

structure/disjoint_set.cpp

+/**
+ * Disjoint-set
+ *
+ * Complexity (Time): O(1)
+ *   makeset   -> O(1)
+ *   find      -> O(a(n))
+ *   union_set -> O(a(n))
+ * Complexity (Space): O(n)
+ */
+
+int par[MAX];
+int rank[MAX];
+int sz[MAX];
+
+// Initialize element x
+void makeset(int x) {
+  par[x] = x;
+  rank[x] = 0;
+  sz[x] = 1;
+}
+
+// Return set index from element x
+int find(int x) {
+  if (par[x] != x)
+    par[x] = find(par[x]);
+
+  return par[x];
+}
+
+// Make x and y belong to the same set
+void union_set(int x, int y) {
+  int xroot = find(x);
+  int yroot = find(y);
+
+  if (xroot == yroot)
+    return;
+
+  if (rank[xroot] < rank[yroot]) {
+    par[xroot] = yroot;
+    sz[yroot] += sz[xroot];
+  } else if (rank[xroot] > rank[yroot]) {
+    par[yroot] = xroot;
+    sz[xroot] += sz[yroot];
+  } else {
+    par[yroot] = xroot;
+    sz[xroot] += sz[yroot];
+    rank[xroot]++;
+  }
+}