Add binary search tree.

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

@@ -9,7 +9,7 @@ export default class BinarySearchTreeNode extends BinaryTreeNode {
       } else {
         this.left = new BinarySearchTreeNode(value);
       }
-    } else {
+    } else if (value > this.value) {
       // Insert to the right.
       if (this.right) {
         this.right.insert(value);
@@ -30,7 +30,7 @@ export default class BinarySearchTreeNode extends BinaryTreeNode {
     if (value < this.value && this.left) {
       // Check left nodes.
       return this.left.contains(value);
-    } else if (this.right) {
+    } else if (value > this.value && this.right) {
       // Check right nodes.
       return this.right.contains(value);
     }

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

@@ -41,4 +41,19 @@ describe('BinarySearchTreeNode', () => {
     expect(bstNode.contains(6)).toBeTruthy();
     expect(bstNode.contains(8)).toBeFalsy();
   });
+
+  it('should not insert duplicates', () => {
+    const bstNode = new BinarySearchTreeNode(2);
+    bstNode.insert(1);
+
+    expect(bstNode.toString()).toBe('1,2');
+    expect(bstNode.contains(1)).toBeTruthy();
+    expect(bstNode.contains(3)).toBeFalsy();
+
+    bstNode.insert(1);
+
+    expect(bstNode.toString()).toBe('1,2');
+    expect(bstNode.contains(1)).toBeTruthy();
+    expect(bstNode.contains(3)).toBeFalsy();
+  });
 });