add BSTNode.isBSTTree() and BSTNode.prototype.findSiblingElem() methods
add BSTNode.isBSTTree() and BSTNode.prototype.findSiblingElem() methods

soliury/data-structure-with-js,LukeLin/js-stl,LukeLin/js-stl,sabrinaluo/data-structure-with-js,LukeLin/data-structure-with-js,LukeLin/data-structure-with-js