void printLevelOrderZigZag(BinaryTree *root) {
   if(!root) return;
   stack<BinaryTree*> S;
   bool leftToRight = true;
   S.push(root);
   int nodesInCurrentLevel = 1;
   int nodesInNextLevel;
   while (!S.empty()) {
      nodesInNextLevel = 0;
      for(int i=0; i<nodesInCurrentLevel; ++i)
      {
          BinaryTree *currNode = S.top();
          S.pop();
          cout << currNode->data << " ";
          if(leftToRight)
          {
              if(currNode->left)
              {
                 S.push(currNode->left);
                 ++nodesInNextLevel;
              }
              if(currNode->right)
              {
                 S.push(currNode->right);
                 ++nodesInNextLevel;
              }
          }
          else
          {
              if(currNode->right)
              {
                 S.push(currNode->right);
                 ++nodesInNextLevel;
              }
              if(currNode->left)
              {
                 S.push(currNode->left);
                 ++nodesInNextLevel;
              }
          }
      }
      cout << endl;
      nodesInCurrentLevel = nodesInNextLevel;
      leftToRight = !leftToRight;
   }
}