Lightweight union find/disjoint set data structure with path compression

UnionFind.js

// Construct a UnionFind object for integer elements 0... n-1
function UnionFind(n) {
    this.s = []; // Underlying array
    this.numSets = n;
    for (var i = 0; i < n; i++) // initial representatives are -1
        this.s[i] = -1
};

// Search for element y and return the index of the root of the tree containing y;
// does path compression on each find
UnionFind.prototype.find = function (y) {
    if (this.s[y] < 0)
        return y;
    else
        return this.s[y] = this.find(this.s[y]);
};

// Form the union of sets containing values x and y using "union by size" strategy
UnionFind.prototype.union = function (x, y) {
    var rootx = this.find(x);
    var rooty = this.find(y);
    if (rootx === rooty)
        return -1;
    this.numSets--;
    var smallroot, bigroot;
    if (this.s[rootx] <= this.s[rooty]) {
        smallroot = rooty;
        bigroot = rootx;
    }
    else {
        smallroot = rootx;
        bigroot = rooty;
    }
    this.s[bigroot] += this.s[smallroot];
    return this.s[smallroot] = bigroot;
};

// Return the size of the tree containing x
UnionFind.prototype.size = function (x) {
    var root = this.find(x);
    return -(this.s[root]);
};

// Make all underlying sets disjoint
UnionFind.prototype.clear = function () {
    for (var i = 0; i < this.s.length; i++)
        this.s[i] = -1
};