algorithm: letter combinations (#1209)

Recursive/test/LetterCombination.test.js

@@ -0,0 +1,48 @@
+import { letterCombinations } from '../LetterCombination'
+
+describe('Letter Combinations', () => {
+  it('should return empty array if provided string is not valid', () => {
+    const result = letterCombinations('')
+    expect(Array.isArray(result)).toBe(true)
+    expect(result.length).toBe(0)
+  })
+
+  it('should return empty array if provided string is empty', () => {
+    const result = letterCombinations(null)
+    expect(Array.isArray(result)).toBe(true)
+    expect(result.length).toBe(0)
+  })
+
+  it('should return letter combination of 234', () => {
+    const result = letterCombinations('234')
+    expect(result).toEqual([
+      'adg',
+      'adh',
+      'adi',
+      'aeg',
+      'aeh',
+      'aei',
+      'afg',
+      'afh',
+      'afi',
+      'bdg',
+      'bdh',
+      'bdi',
+      'beg',
+      'beh',
+      'bei',
+      'bfg',
+      'bfh',
+      'bfi',
+      'cdg',
+      'cdh',
+      'cdi',
+      'ceg',
+      'ceh',
+      'cei',
+      'cfg',
+      'cfh',
+      'cfi'
+    ])
+  })
+})