Fix and Improvement

[prithvi007]

1. Fixed output issue with zigzagLevelOrder function
   As the expected result, as mentioned above in the file was:
   [
   [3],
   [20,9],
   [15,7]
   ]

   The function was returning the result as:
   [
   [3],
   [9,20],
   [7,15]
   ]

2. I was struggling myself to know the input for the zigzagLevelOrder function.
   Found it at last. And wanted to contribute it. Thought it would be helpful to others.
   So I added a input variable binaryTree with the value, which is an input to zigzagLevelOrder function,
   And also consoled the result with an index.html file, so that people can just clone the repo and see the result in browser
   console quickly and play around.

3. Update the zigzagLevelOrder function to arrow function and change var to const declaration.