247. Strobogrammatic Number II

给定n，返回所有长度为n的频闪数字。

• 时间复杂度 O(n)，空间复杂度O(n)
• Runtime: 140 ms, faster than 95.92%
• Memory Usage: 61.2 MB, less than 61.90%

/**
 * @param {number} n
 * @return {string[]}
 */
var findStrobogrammatic = function(n) {
  const helper = (n, m) => {
    if (n === 0) return [''];
    if (n === 1) return ['0', '1', '8'];
    const list = helper(n - 2, m);
    const res = [];
    for (let i = 0; i < list.length; i++) {
      const s = list[i];
      if (n !== m) res.push('0' + s + '0');
      res.push('1' + s + '1');
      res.push('6' + s + '9');
      res.push('8' + s + '8');
      res.push('9' + s + '6');
    }
    return res;
  }
  return helper(n, n);
};