JavaScript: Letter combinations of a number

JavaScript Function: Exercise-13 with Solution

A string consists of digits ranging from 2 to 9, inclusive. Write a JavaScript program to get all possible letter combinations that represent the number using recursion.

Letter Combination:
1: 'abc',
2: 'def',
3: 'ghi',
4: 'jkl',
5: 'mno',
6: 'pqr',
7: 'stu',
8: 'vwx'
9: 'yz'

Test Data:
("12") -> ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"]
("9") -> ["y", "z"]

Sample Solution-1:

JavaScript Code:

// Function to generate letter combinations for a given digit string
const letterCombinations = (digits) => {
  const length = digits?.length; // Get the length of the input digits
  const result = []; // Initialize an array to store the combinations

  // Base case: If the input digits are empty, return an empty array
  if (!length) {
    return result;
  }

  // Map of digits to corresponding letters on a phone keypad
  const digitMap = {
    1: 'abc',
    2: 'def',
    3: 'ghi',
    4: 'jkl',
    5: 'mno',
    6: 'pqr',
    7: 'stu',
    8: 'vwx',
    9: 'yz',
  };

  // Recursive function to generate combinations
  const generateCombinations = (index, combination) => {
    let letter;
    let letterIndex;

    // If the index exceeds the length, add the combination to the result array
    if (index >= length) {
      result.push(combination);
      return;
    }

    // Get the letters corresponding to the current digit
    const digit = digitMap[digits[index]];

    // Iterate through each letter and recursively generate combinations
    letterIndex = 0;
    while ((letter = digit[letterIndex++])) {
      generateCombinations(index + 1, combination + letter);
    }
  };

  // Start generating combinations from the first digit
  generateCombinations(0, '');

  // Return the final array of combinations
  return result;
};

// Test the function with different input digits
console.log(letterCombinations("12")); // Output: ['ad', 'ae', 'af', 'bd', 'be', 'bf', 'cd', 'ce', 'cf']
console.log(letterCombinations("9"));  // Output: ['yw', 'yx', 'yy', 'zw', 'zx', 'zy']

Output:

["ad","ae","af","bd","be","bf","cd","ce","cf"]
["y","z"]

Flowchart:

Live Demo:

See the Pen javascript-recursion-function-exercise-13 by w3resource (@w3resource) on CodePen.

Sample Solution-2:

JavaScript Code:

const letterCombinations = (digits) => {
  const digitMap = {
    1: 'abc',
    2: 'def',
    3: 'ghi',
    4: 'jkl',
    5: 'mno',
    6: 'pqr',
    7: 'stu',
    8: 'vwx',
    9: 'yz',
  };

  const backtrack = (index, currentCombination) => {
    if (index === digits.length) {
      result.push(currentCombination);
      return;
    }

    const currentDigit = digits[index];
    const letters = digitMap[currentDigit];

    for (const letter of letters) {
      backtrack(index + 1, currentCombination + letter);
    }
  };

  const result = [];
  if (digits) {
    backtrack(0, '');
  }

  return result;
};

// Test the function with different input digits
console.log(letterCombinations("12")); // Output: ['ad', 'ae', 'af', 'bd', 'be', 'bf', 'cd', 'ce', 'cf']
console.log(letterCombinations("9"));  // Output: ['yw', 'yx', 'yy', 'zw', 'zx', 'zy']

Output:

["ad","ae","af","bd","be","bf","cd","ce","cf"]
["y","z"]

Flowchart:

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