Construct a string consists of parenthesis and integers from a binary tree with the preorder traversing way.
The null node needs to be represented by empty parenthesis pair “()”. Omit all the empty parenthesis pairs that don’t affect the one-to-one mapping relationship between the string and the original binary tree.
Binary tree to string with brackets : https://www.geeksforgeeks.org/binary-tree-string-brackets/