root(element) {
    this._introduce(element);
    if (this._parents[element] !== element) {
      this._parents[element] = this.root(this._parents[element]);
    }
    return this._parents[element];
  }
Return the root element which represents the given element's subset.
The result does not depend on the choice of the element,
  but rather on the subset itself.
Complexity: O(A^-1) (inverse Ackermann function) amortized.

@param {*} element
@return {*}