Method _findParentForNode

src/cdk/tree/tree.ts:977–994

* Finds the parent for the given node. If this is a root node, this * returns null. If we're unable to determine the parent, for example, * if we don't have cached node data, this returns undefined.

(node: T, index: number, cachedNodes: readonly T[])

   * if we don't have cached node data, this returns undefined.
   */
  private _findParentForNode(node: T, index: number, cachedNodes: readonly T[]): T | null {
    // In all cases, we have a mapping from node to level; all we need to do here is backtrack in
    // our flattened list of nodes to determine the first node that's of a level lower than the
    // provided node.
    if (!cachedNodes.length) {
      return null;
    }
    const currentLevel = this._levels.get(this._getExpansionKey(node)) ?? 0;
    for (let parentIndex = index - 1; parentIndex >= 0; parentIndex--) {
      const parentNode = cachedNodes[parentIndex];
      const parentLevel = this._levels.get(this._getExpansionKey(parentNode)) ?? 0;

      if (parentLevel < currentLevel) {
        return parentNode;
      }
    }
    return null;
  }

  /**
   * Given a set of root nodes and the current node level, flattens any nested