UnionFind1.js

function UnionFind(n) {
    this.id = [...Array(n)].map((_, i) => i)
    this.sz = [...Array(n)].map(_ => 1)
    this.max = 0
}

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]
    } else {
        this.id[iroot] = kroot
        this.sz[kroot] += this.sz[iroot]
    }

    this.max = Math.max(this.max, this.sz[iroot], this.sz[kroot])
}

UnionFind.prototype.connected = function (i, k) {
    return this._root(i) === this._root(k)
}

UnionFind.prototype.unitary = function () {
    return this.max === this.id.length
}

l = console.log

var qu = new UnionFind(5)
qu.union(1,2)
l(qu.unitary()) // false
qu.union(3,4)
l(qu.unitary()) // false
qu.union(1,4)
l(qu.unitary()) // false
qu.union(0,3)
console.log(qu.unitary()) // true