UnionFind3.js

function UnionFind(n) {
    this.id = [...Array(n)].map((_, i) => i)
    this.sz = [...Array(n)].map(_ => 1)
    this.actual = [...this.id]
}

UnionFind.prototype._root = function (id) {
    while(id != this.id[id])
        id = this.id[id]
    return id
}

UnionFind.prototype.union = function (i, k) {
    let iroot = this._root(i)
    let kroot = this._root(k)

    if (iroot === kroot) return

    if (this.sz[iroot] > this.sz[kroot]) {
        this.id[kroot] = iroot
        this.sz[iroot] += this.sz[kroot]
        this.actual[iroot] = kroot
    } else {
        this.id[iroot] = kroot
        this.sz[kroot] += this.sz[iroot]
    }
}

UnionFind.prototype.delete = function (x) {
    this.union(x, x + 1)
}

UnionFind.prototype.after = function (x) {
    return this.actual[this._root(x + 1)]
}