https://hellohasan.com/2017/01/15/%e0%a6%9f%e0%a7%8d%e0%a6%b0%e0%a6%bf-%e0%a6%b8%e0%a7%8d%e0%a6%9f%e0%a7%8d%e0%a6%b0%e0%a6%be%e0%a6%95%e0%a6%9a%e0%a6%be%e0%a6%b0-binary-search-tree-traversal/