Add binary search tree.

src/data-structures/tree/binary-search-tree/BinarySearchTreeNode.js

@@ -0,0 +1,38 @@
+import BinaryTreeNode from '../BinaryTreeNode';
+
+export default class BinarySearchTreeNode extends BinaryTreeNode {
+  insert(value) {
+    if (value < this.value) {
+      // Insert to the left.
+      if (this.left) {
+        this.left.insert(value);
+      } else {
+        this.left = new BinarySearchTreeNode(value);
+      }
+    } else {
+      // Insert to the right.
+      if (this.right) {
+        this.right.insert(value);
+      } else {
+        this.right = new BinarySearchTreeNode(value);
+      }
+    }
+
+    return this;
+  }
+
+  contains(value) {
+    // Check the root.
+    if (this.value === value) {
+      return true;
+    }
+
+    if (value < this.value && this.left) {
+      return this.left.contains(value);
+    } else if (this.right) {
+      return this.right.contains(value);
+    }
+
+    return false;
+  }
+}