int sumLeaves(BST& t) {
 // PRE: t is a Binary Seacrch Tree
 // POST: outputs the sum of the items in the leaves only.
 
 // You will need to use the methods getLeftSubTree, getRightSubtree, isLeaf
 // and getRootItem defined in the file extendedBST.cpp.
 // Do not implement a function "int main().." in this file as it will confuse the
 // automarker.
 
 // complete the code.
  
}