const assert = require('assert');

/**
 * 9宫格数字序列对应的字母组合
 * @param digits
 */
function letterCombinations(digits) {
  const letterMap = {
    0: ' ',
    1: '',
    2: 'abc',
    3: 'def',
    4: 'ghi',
    5: 'jkl',
    6: 'mno',
    7: 'pqrs',
    8: 'tuv',
    9: 'wxys'
  };

  const resCombs = [];

  /**
   *
   *  寻找和 digits[index]匹配的字母，获得digits[0...index]翻译得到的解
   * @param digits
   * @param index
   * @param s 保存了此时从 digits[0...index-1] 翻译得到的一个字母字符串
   */
  function findCombination(digits, index, s) {
    if (index === digits.length) {
      resCombs.push(s);
      return;
    }

    const char = digits[index];
    assert(char >= '0' && char <= '9' && char !== '1');
    const letters = letterMap[char];
    for (let i = 0; i < letters.length; i++) {
      findCombination(digits, index + 1, s + letters[i]);
    }

    return;
  }


  // 计算逻辑
  if (digits === '') return resCombs;

  findCombination(digits, 0, '');

  return resCombs;
}

console.log(letterCombinations('045'));