public void addLeafNode(LeafNode newNode) {
    if( root == null ) {
      root = newNode;
    }
    nodeStack.push(newNode);
  }
Adds a leaf node, pushes the new node onto the stack.
@param newNode the new node
standalone-metastore/metastore-server/src/main/java/org/apache/hadoop/hive/metastore/parser/ExpressionTree.java