How to convert a binary tree to binary search tree in-place, i.e., we cannot use any extra space

Question

How to convert a binary tree to binary search tree in-place, i.e., we cannot use any extra space.

Answer 1

Well, if this is an interview question, the first thing I'd blurt out (with zero actual thought) is this: iterate the entire binary recursively and and find the smallest element. Take it out of the binary tree. Now, repeat the process where you iterate the entire tree and find the smallest element, and add it as a parent of the last element found (with the previous element becoming the new node's left child). Repeat as many times as necessary until the original tree is empty. At the end, you are left with the worst possible sorted binary tree -- a linked list. Your pointer is pointing to the root node, which is the largest element.

This is a horrible algorithm all-around - O(n^2) running time with the worst possible binary tree output, but it's a decent starting point before coming up with something better and has the advantage of you being able to write the code for it in about 20 lines on a whiteboard.