https://leetcode.com/problems/decode-ways/

Leetcode 91. Decode Ways.js

var numDecodings = function(s) {
    
    const memo = new Array(s.length).fill(null); // for performance

    const inner = (i) => {
        // base cases
        if(i === s.length) return 1; // if we make it to the end, we've completed a valid decoding
        if(s[i] === '0') return 0; // no decoding can have a leading 0
        
        if(memo[i] !== null) return memo[i]; // for performance

        if(i < s.length-1 && parseInt(`${s[i]}${s[i+1]}`, 10) < 27){ // if there are two ways to decode
            return memo[i] = inner(i+1) + inner(i+2); // do both
        }else{
            return memo[i] = inner(i+1); // otherwise just do the one
        }
    };
    
    return inner(0);
};