A 2tree is a tree in which each vertex either has exactly 2 nonempty
children (its left and right child),
or is a leaf (has no children). Each leaf is named with an uppercase letter,
and each other vertex is named with a lowercase letter.
We want to mutate a given 2tree by swapping the locations of two of its subtrees.
For example, below is shown a 2tree and then its mutation when its
subtrees rooted at C and x are swapped. q q
x z ==> C z
A B C g x g
R Q A B R Q
Each 2tree can be represented by a string consisting of the names of its
vertices in the order given by a postorder traversal of the tree (see notes).
Given tree, the representation of a 2tree, and the 0based indices of two of its vertices,
return the representation of the mutated tree. If the two subtrees have
any vertices in common return the 7character string "OVERLAP". If tree is
not the representation of any 2tree, return "BADTREE".
