/**
 * @param {string} s
 * @return {number}
 */
var numDecodings = function(s) {
    if (s.length === 0) return 0;
    var dp = [1];
    s[0] === '0' ? dp[1] = 0 : dp[1] = 1;

    for (var i = 2; i <= s.length; i++) {
        var prevTwo = parseInt(s.substring(i - 2, i));
        dp[i] = 0;
        if (s[i - 1] !== '0') dp[i] = dp[i - 1];
        if (prevTwo >= 10 && prevTwo <= 26 ) dp[i] += dp[i - 2];
    }

    return dp[s.length];
};