Day 179

Good morning! Here's your coding interview problem for today.

This problem was asked by Google.

Given the sequence of keys visited by a postorder traversal of a binary search tree, reconstruct the tree.

For example, given the sequence `2, 4, 3, 8, 7, 5`, you should construct the following tree:
```
    5
   / \
  3   7
 / \   \
2   4   8
```