91.decode-ways.ts

// dp [i] = dp[i+1] + dp[i+2]
function numDecodings(s: string): number {
  const dp = { [s.length]: 1 };

  for (let i = s.length - 1; i >= 0; i--) {
    if (s[i] === "0") {
      dp[i] = 0;
    } else {
      dp[i] = dp[i + 1];
    }

    if (
      i + 1 < s.length &&
      (s[i] === "1" || (s[i] === "2" && "0123456".includes(s[i + 1])))
    ) {
      dp[i] = dp[i] + dp[i + 2];
    }
  }

  return dp[0];
}

console.log(numDecodings("226"));
console.log(numDecodings("106"));