LCA implementation (fixes #89) (#91)

* Create solution file

* Move to correct folder

* Add the algorithm

Ref: https://www.geeksforgeeks.org/lowest-common-ancestor-in-a-binary-search-tree/

* Add a test case

* Fix test case

* Fix test by adding missing arguments

* Move to inside BST folder

* Move to inside BST folder

* Add more tests and use BST class

* Fix variable declaration

* Update reference

* Match naming used in BST

Author: amahdy

src/_DataStructures_/Trees/BinarySearchTree/lowest-common-ancestor/index.js

@@ -0,0 +1,24 @@
+/**
+ * Lowest Common Ancestor in a Binary Search Tree.
+ *
+ * Given values of two values n1 and n2 in a Binary Search Tree, find the Lowest Common Ancestor (LCA). You may assume that both the values exist in the tree.
+ */
+ 
+function lca(node, n1, n2) {
+  if (node == null)
+    return null;
+
+  // If both n1 and n2 are smaller than root, then LCA lies in left
+  if (node.value > n1 && node.value > n2)
+    return lca(node.leftChild, n1, n2);
+
+  // If both n1 and n2 are greater than root, then LCA lies in right
+  if (node.value < n1 && node.value < n2)
+    return lca(node.rightChild, n1, n2);
+
+  return node;
+}
+
+module.exports = {
+  lca,
+};

src/_DataStructures_/Trees/BinarySearchTree/lowest-common-ancestor/index.test.js

@@ -0,0 +1,28 @@
+const { lca } = require('.');
+const BinarySearchTree = require('../index');
+
+// Quick JSON equivalent
+// {"left":{"left":{"data":4},"right":{"left":{"data":10},"right":{"data":14},"data":12},"data":8},"right":{"data":22},"data":20}
+
+describe('LCA', () => {
+
+  let bst = new BinarySearchTree(20);
+  bst.add(22);
+  bst.add(8);
+  bst.add(12);
+  bst.add(4);
+  bst.add(14);
+  bst.add(10);
+
+  it('Should return 12', () => {
+    expect(lca(bst.root, 10, 14).value).toEqual(12);
+  });
+  
+  it('Should return 8', () => {
+    expect(lca(bst.root, 14, 8).value).toEqual(8);
+  });
+  
+  it('Should return 20', () => {
+    expect(lca(bst.root, 10, 22).value).toEqual(20);
+  });
+});