function lcs(a: string, b: string): number {
  const rows = a.length + 1;
  const cols = b.length + 1;
  const dp = Array.from({ length: rows }, () => new Array<number>(cols).fill(0));
  for (let i = 1; i < rows; i++) {
    for (let j = 1; j < cols; j++) {
      if (a[i - 1] === b[j - 1]) {
        dp[i][j] = dp[i - 1][j - 1] + 1;
      } else {
        dp[i][j] = Math.max(dp[i - 1][j], dp[i][j - 1]);
      }
    }
  }
  return dp[rows - 1][cols - 1];
}