压缩字符串

function encode(str) {
  const l = [];
  let i = -1;
  let lastChar;
  for (const char of str) {
    if (char !== lastChar) {
      lastChar = char;
      i++;
      l[i] = [char, 1];
    } else {
      l[i][1]++;
    }
  }
  return l.flat().join("");
}