CodeFights Solutions

expand_it.js

// https://codefights.com/challenge/wwPuroz78u6yA8cGD

function Expand_It(s, k) {
    return s.match(/\w\d+/g).sort().reduce(function (a, b) {
        return a + b[0].repeat(+b.slice(1));
    }, '')[k - 1] || '-1';
}

function Expand_It(s, k) {
    return s.match(/.\d+/g).sort().find(function (a) {
      return (k -= a.slice(1)) < 1;
    })[0];
}

function Expand_It(s, k) {
   e = s.match(/\w\d+/g).sort()
   for (v in e)
       if ((k -= e[v].slice(1)) <= 0) return e[v][0]
}

Expand_It = (s, k) => s.match(/.\d+/g).sort().find(a => (k -= a.slice(1)) < 1)[0]

Expand_It('z2b13a5c2', 10); // b

sumitup.js

https://codefights.com/challenge/z6jikHZ7iumoDB86m/main

a = (n, d) => d ? n * a(--n, --d) : 1

SumItUp = s = (n, k, d) => {
    if (!d) d = n + 1
    return k ? a(++n, d) + s(n, --k, d) : 0
}