advent-code-2021-day-12.js

let input = `fs-end
he-DX
fs-he
start-DX
pj-DX
end-zg
zg-sl
zg-pj
pj-he
RW-he
fs-DX
pj-RW
zg-RW
start-pj
he-WI
zg-he
pj-fs
start-RW`;

function isSmall(str) {
  return str.toLowerCase() === str;
}

function run() {
  const lines = input.split("\n").filter(Boolean);
  /**
   * @type {Record<string, string[]>}
   */
  const fromTo = lines
    .flatMap((line) => {
      const [from, to] = line.split("-");
      return [
        [from, to],
        [to, from],
      ];
    })
    .filter(([from, to]) => to !== "start")
    .reduce((acc, [from, to]) => {
      acc[from] = acc[from] || [];
      acc[from].push(to);
      return acc;
    }, {});

  function toEnd(next = "start", visited = new Set(), twiceFlag = false) {
    if (isSmall(next) && visited.has(next) && twiceFlag) {
      return [];
    }
    if (next === "end") {
      return [["end"]];
    }
    if (isSmall(next)) {
      visited = new Set(visited);
      if (visited.has(next)) {
        twiceFlag = true;
      } else {
        visited.add(next);
      }
    }

    return fromTo[next]
      .flatMap((to) => toEnd(to, visited, twiceFlag))
      .map((subPath) => [next, ...subPath]);
  }

  const res = toEnd().map((line) => line.join(" - "));
  console.log(res, res.length);
}

run();