Add binary search tree.

src/data-structures/tree/BinaryTreeNode.js

@@ -0,0 +1,37 @@
+export default class BinaryTreeNode {
+  constructor(value = null, left = null, right = null) {
+    this.left = left;
+    this.right = right;
+    this.value = value;
+  }
+
+  addLeft(node) {
+    this.left = node;
+    return this;
+  }
+
+  addRight(node) {
+    this.right = node;
+    return this;
+  }
+
+  hasLeft() {
+    return !!this.left;
+  }
+
+  hasRight() {
+    return !!this.right;
+  }
+
+  traverseInOrder() {
+    return Array.prototype.concat(
+      this.left ? this.left.traverseInOrder() : [null],
+      [this.value],
+      this.right ? this.right.traverseInOrder() : [null],
+    );
+  }
+
+  toString() {
+    return this.traverseInOrder().filter(value => !!value).toString();
+  }
+}