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#!/usr/local/bin/node
//
// See ../README.md
//
//
// Run as: node ch-2.js < input-file
//
require ('readline')
. createInterface ({input: process . stdin})
. on ('line', (num) => {
let number = "0";
for (let i = 0; i < + num; i ++) {
number = next_number (number)
}
console . log (number)
})
function next_number (prev_number) {
//
// Grab the trailing 3s (tail), its preceding number (num), and
// anything before that (prefix).
//
let [match, prefix, num, tail] =
("0" + prev_number) . match (/^(.*)([012])(3*)$/)
//
// First, we take the prefix, and add (num + 1) to it,
// then the tail, where we have replaced each 3 by a 1.
// Then replace any '11' with '12' (we can only have 11s at the end).
// Finally, remove any leading 0.
//
return (prefix + (+ num + 1) + (tail . replace (/3/g, "1"))) .
replace (/11/g, "12") .
replace (/^0/, "")
}
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