-> For 1000 elements, except last level in tree, sum of total number of elements for all levels will be 511.
reason for above statement: In binary tree for every next level elements gets doubled and sum of elements of all levels except the last level will be nearest 2's power - 1.
here, nearest 2's power from 1000 is 2^9 = 512. So total no of element in tree except last level = 512 - 1 = 511.
hence index of largest element will 511, here it will be 510.(because indexing starts from 0).
Now if draw a binary search tree with 2 , 3 or any level, and write down the inorder traversal, you will observe 2nd last node always stores the 3rd largest element.
Hence, in this case index of 3rd largest element(which stored at 2nd last node in tree) = 510 (last node index) -1 = 509.