decodeway

solution.js

const map = 'abcdefghijklmnopqrstuvwxyz'
  .split('')
  .reduce((obj, curr, i) => ({
    ...obj,
      [String(i+1)]: curr.toUpperCase()
  }), {})

/**
 * @param {string} s
 * @return {number}
 */
var numDecodings = function(s) {
    let decodings = [{head:'',tail:s}];
    let count = 0
    const seen = {}
    while (decodings.length) {
      const next = [];
      for (const d of decodings) {
        // already done
        if (seen[d.head]) continue
        if (!d.tail.length) {
          count++
          seen[d.head] = true
          continue;
        }
        const r = decode(d);
        next.push(...r.results);
      }
      decodings = next
    }

    return count
};


function decode(obj) {
    const results = [];
    const a = obj.tail[0];
    const b = obj.tail[1];
    const comb = [a];
    if (a === '0') {
     return { valid: false, results }
    }
    if (b) {
      comb.push(a+b)
    }
    for (const c of comb) {
     if (map.hasOwnProperty(c)) {
       results.push({
         head: obj.head + map[c],
         tail: obj.tail.slice(c.length)
       })
     }
    }
    return { valid: true, results }
}