algorithm: letter combinations (#1209)

Author: hiitesh1127

Diff for: DIRECTORY.md

@@ -252,6 +252,7 @@
   * [FibonacciNumberRecursive](Recursive/FibonacciNumberRecursive.js)
   * [FloodFill](Recursive/FloodFill.js)
   * [KochSnowflake](Recursive/KochSnowflake.js)
+  * [LetterCombination](Recursive/LetterCombination.js)
   * [Palindrome](Recursive/Palindrome.js)
   * [SubsequenceRecursive](Recursive/SubsequenceRecursive.js)
   * [TowerOfHanoi](Recursive/TowerOfHanoi.js)

Diff for: Recursive/LetterCombination.js

@@ -0,0 +1,53 @@
+/*
+ *
+ *  Letter Combinations of a Phone Number
+ *
+ *  Given a string containing digits from 2-9 inclusive,
+ *  return all possible letter combinations that the number could represent.
+ *  Return the answer in any order.
+
+ *  A mapping of digits to letters (just like on the telephone buttons) is given below.
+ *  Note that 1 does not map to any letters.
+ *  More info: https://leetcode.com/problems/letter-combinations-of-a-phone-number/
+ */
+
+/*
+ * @param {string} digits
+ * @returns {string[]} all the possible combinations
+ */
+
+const letterCombinations = (digits) => {
+  const length = digits?.length
+  const result = []
+  if (!length) {
+    return result
+  }
+  const digitMap = {
+    2: 'abc',
+    3: 'def',
+    4: 'ghi',
+    5: 'jkl',
+    6: 'mno',
+    7: 'pqrs',
+    8: 'tuv',
+    9: 'wxyz'
+  }
+
+  const combinations = (index, combination) => {
+    let letter
+    let letterIndex
+    if (index >= length) {
+      result.push(combination)
+      return
+    }
+    const digit = digitMap[digits[index]]
+    letterIndex = 0
+    while ((letter = digit[letterIndex++])) {
+      combinations(index + 1, combination + letter)
+    }
+  }
+  combinations(0, '')
+  return result
+}
+
+export { letterCombinations }

Diff for: 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'
+    ])
+  })
+})