find(itemValue) {
    const templateDisjointItem = new DisjointSetItem(itemValue, this.keyCallback);

    // Try to find item itself;
    const requiredDisjointItem = this.items[templateDisjointItem.getKey()];

    if (!requiredDisjointItem) {
      return null;
    }

    return requiredDisjointItem.getRoot().getKey();
  }
Find set representation node.

@param {*} itemValue
@return {(string|null)}