Done with combination task.

Author: simdi

combination/index.js

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+/* 
+* @author: Chisimdi Damian Ezeanieto
+* @data: 11/03/2023
+*/
+
+// Given an input '23', return all possible character match in a key pad match
+// e.g input: '23', output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]
+const phoneDigits = {
+  "2": ['a', 'b', 'c'],
+  "3": ['d', 'e', 'f'],
+  "4": ['g', 'h', 'i'],
+  "5": ['j', 'k', 'l'],
+  "6": ['m', 'n', 'o'],
+  "7": ['p', 'q', 'r', 's'],
+  "8": ['t', 'u', 'v'],
+  "9": ['w', 'x', 'y', 'z'],
+};
+// const generateCombinations = (digit) => {
+//   const firstDigit = digit[0];
+//   const remainingDigit = digit.substring(1);
+
+//   let chars = [];
+//   phoneDigits[firstDigit].map(firstChar => {
+//     for (let i = 0; i < remainingDigit.length; i++) {
+//       let remainingPhoneDigit = phoneDigits[remainingDigit[i]];
+
+//       const combination = remainingPhoneDigit.map(comb => firstChar + comb);
+//       chars.push(combination);
+//     }
+//   });
+//   return chars.flatMap(x => x);
+// };
+
+// Another solution
+// const generateCombinations = (digits) => {
+//   let combinations = [''];
+  
+//   for (let digit of digits) {
+//     const letters = phoneDigits[digit];
+//     const newCombinations = [];
+    
+//     for (let combination of combinations) {
+//       for (let letter of letters) {
+//         newCombinations.push(combination + letter);
+//       }
+//     }
+    
+//     combinations = newCombinations;
+//   }
+  
+//   return combinations;
+// }
+
+// Using recursion to solve this generate combination problem.
+const generateCombinations = (digit) => {
+  // define base class
+  if (digit.length === 0) return [''];
+
+  const firstChar = digit[0];
+  const remainingChars = digit.slice(1);
+
+  const result = phoneDigits[firstChar].flatMap(x => {
+    const combination = generateCombinations(remainingChars);
+    return combination.map(comb => x + comb);
+  });
+
+  return result;
+};
+
+module.exports = generateCombinations;

combination/test.js

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+/*
+* @author: Chisimdi Damian Ezeanieto
+* @date: 11/03/2023
+*/
+const generateCombinations = require('./index');
+
+describe('Phone key pad match', () => {
+  test('generateCombinations function is defined', () => {
+    expect(typeof generateCombinations).toEqual('function');
+  });
+  test('Calculate correct generateCombinations value for "23" === ["ad","ae","af","bd","be","bf","cd","ce","cf"]', () => {
+    expect(generateCombinations('23')).toEqual(["ad","ae","af","bd","be","bf","cd","ce","cf"]);
+  });
+  test("Calculate correct generateCombinations value for '234' === ['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']", () => {
+    expect(generateCombinations('234')).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']);
+  });
+  test("Calculate correct generateCombinations value for 'aaaa' & 'aa' === ['dg', 'dh', 'di', 'eg', 'eh', 'ei', 'fg', 'fh', 'fi']", () => {
+    expect(generateCombinations('34')).toEqual(['dg', 'dh', 'di', 'eg', 'eh', 'ei', 'fg', 'fh', 'fi']);
+  });
+});