nhistory.js

/**
 * @param {string} s
 * @return {number}
 */
var numDecodings = function (s) {
  // Edge case
  if (s.length === 0 || s[0] === "0") return 0;

  let dp = new Array(s.length + 1).fill(0);

  dp[0] = 1;
  dp[1] = 1;

  for (let i = 2; i <= s.length; i++) {
    let single = s[i - 1];
    let double = s[i - 2] + s[i - 1];

    if (single >= 1 && single <= 9) dp[i] += dp[i - 1];
    if (double >= 10 && double <= 26) dp[i] += dp[i - 2];
  }

  return dp[s.length];
};

// TC: O(n)
// SC: O(n)