0091-decode-ways.ts

function numDecodings(s: string): number {
    let dp = {
        [s.length]: 1,
    };

    for (let i = s.length - 1; i > -1; 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 + 2];
        }
    }
    return dp[0];
}