var countDigitOne = function(n) {
  let count = 0;
  for (let i = n; i > 0; i--) {
    const match = i.toString().match(/1/g);
    if (match && match.length > 0) {
      count += match.length;
    }
  }

  return count;
};

const a = (n / 10) * 1 + (n % 10) >= 1 ? 1 : 0;

const b = (n / 100) * 10 + if (k > 19) 10 else if (k < 10) 0 else k - 10 + 1;

const c = (n / 1000) * 100 + if (k > 199) 10 else if (k < 100) 0 else k - 100 + 1;

const k = n % (i * 10)

const d = (n / (i * 10)) * i + if (k > 2 * i - 1) i else if (k < i) 0 else k - i + 1;

const countDigitOne = function(n) {
  let count = 0;

  for (let i = 1; i <= n; i *= 10) {
    // 除数
    let divisor = i * 10;
    // 整除部分
    let divided = Math.floor(n / divisor),
      // 余数
      remainder = n % divisor,
      rest = 0;

    count += divided * i;

    rest = remainder > 2 * i - 1 ? i : remainder < i ? 0 : remainder - i + 1;

    count += rest;
  }

  return count;
};