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algorithm: letter combinations (#1209)

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`252`	`252`	`*[FibonacciNumberRecursive](Recursive/FibonacciNumberRecursive.js)`
`253`	`253`	`*[FloodFill](Recursive/FloodFill.js)`
`254`	`254`	`*[KochSnowflake](Recursive/KochSnowflake.js)`
	`255`	`+*[LetterCombination](Recursive/LetterCombination.js)`
`255`	`256`	`*[Palindrome](Recursive/Palindrome.js)`
`256`	`257`	`*[SubsequenceRecursive](Recursive/SubsequenceRecursive.js)`
`257`	`258`	`*[TowerOfHanoi](Recursive/TowerOfHanoi.js)`

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	`1`	`+/*`
	`2`	`+ *`
	`3`	`+ * Letter Combinations of a Phone Number`
	`4`	`+ *`
	`5`	`+ * Given a string containing digits from 2-9 inclusive,`
	`6`	`+ * return all possible letter combinations that the number could represent.`
	`7`	`+ * Return the answer in any order.`
	`8`	`+`
	`9`	`+ * A mapping of digits to letters (just like on the telephone buttons) is given below.`
	`10`	`+ * Note that 1 does not map to any letters.`
	`11`	`+ * More info: https://leetcode.com/problems/letter-combinations-of-a-phone-number/`
	`12`	`+ */`
	`13`	`+`
	`14`	`+/*`
	`15`	`+ *@param {string} digits`
	`16`	`+ *@returns {string[]} all the possible combinations`
	`17`	`+ */`
	`18`	`+`
	`19`	`+constletterCombinations=(digits)=>{`
	`20`	`+constlength=digits?.length`
	`21`	`+constresult=[]`
	`22`	`+if(!length){`
	`23`	`+returnresult`
	`24`	`+}`
	`25`	`+constdigitMap={`
	`26`	`+2:'abc',`
	`27`	`+3:'def',`
	`28`	`+4:'ghi',`
	`29`	`+5:'jkl',`
	`30`	`+6:'mno',`
	`31`	`+7:'pqrs',`
	`32`	`+8:'tuv',`
	`33`	`+9:'wxyz'`
	`34`	`+}`
	`35`	`+`
	`36`	`+constcombinations=(index,combination)=>{`
	`37`	`+letletter`
	`38`	`+letletterIndex`
	`39`	`+if(index>=length){`
	`40`	`+result.push(combination)`
	`41`	`+return`
	`42`	`+}`
	`43`	`+constdigit=digitMap[digits[index]]`
	`44`	`+letterIndex=0`
	`45`	`+while((letter=digit[letterIndex++])){`
	`46`	`+combinations(index+1,combination+letter)`
	`47`	`+}`
	`48`	`+}`
	`49`	`+combinations(0,'')`
	`50`	`+returnresult`
	`51`	`+}`
	`52`	`+`
	`53`	`+export{letterCombinations}`

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	`1`	`+import{letterCombinations}from'../LetterCombination'`
	`2`	`+`
	`3`	`+describe('Letter Combinations',()=>{`
	`4`	`+it('should return empty array if provided string is not valid',()=>{`
	`5`	`+constresult=letterCombinations('')`
	`6`	`+expect(Array.isArray(result)).toBe(true)`
	`7`	`+expect(result.length).toBe(0)`
	`8`	`+})`
	`9`	`+`
	`10`	`+it('should return empty array if provided string is empty',()=>{`
	`11`	`+constresult=letterCombinations(null)`
	`12`	`+expect(Array.isArray(result)).toBe(true)`
	`13`	`+expect(result.length).toBe(0)`
	`14`	`+})`
	`15`	`+`
	`16`	`+it('should return letter combination of 234',()=>{`
	`17`	`+constresult=letterCombinations('234')`
	`18`	`+expect(result).toEqual([`
	`19`	`+'adg',`
	`20`	`+'adh',`
	`21`	`+'adi',`
	`22`	`+'aeg',`
	`23`	`+'aeh',`
	`24`	`+'aei',`
	`25`	`+'afg',`
	`26`	`+'afh',`
	`27`	`+'afi',`
	`28`	`+'bdg',`
	`29`	`+'bdh',`
	`30`	`+'bdi',`
	`31`	`+'beg',`
	`32`	`+'beh',`
	`33`	`+'bei',`
	`34`	`+'bfg',`
	`35`	`+'bfh',`
	`36`	`+'bfi',`
	`37`	`+'cdg',`
	`38`	`+'cdh',`
	`39`	`+'cdi',`
	`40`	`+'ceg',`
	`41`	`+'ceh',`
	`42`	`+'cei',`
	`43`	`+'cfg',`
	`44`	`+'cfh',`
	`45`	`+'cfi'`
	`46`	`+])`
	`47`	`+})`
	`48`	`+})`

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