Function find

javascript/0261-graph-valid-tree.js:127–136

(union, i, num = union[i])

const compress = (union, i, head) => (union[i] = head);

const find = (union, i, num = union[i]) => {
    const isEmpty = num === -1;
    if (isEmpty) return i;

    const head = find(union, num);

    compress(union, i, head);

    return union[i];
};