protected final int getSizeOfAncestor() {
    return ancestorStack.size();
  }
Get level from root to NextMatchedNode. Level of root is 0. For example, root.sg.d1.s1,
NextMatchedNode is s1, then return 3.

@return level from root to NextMatchedNode
iotdb-core/node-commons/src/main/java/org/apache/iotdb/commons/schema/tree/AbstractTreeVisitor.java