L Question: Reverse the tree

Bottom up approach:
Although the code for the top-down approach seems concise, it is actually subtle and
there are a lot of hidden traps if you are not careful. The other approach is thinking
recursively in a bottom-up fashion. If we reassign the bottom-level nodes before the
upper ones, we won’t have to make copies and worry about overwriting something. We
know the new root will be the left-most leaf node, so we begin the reassignment here.

public TreeNode UpsideDownBinaryTree(TreeNode root) {
return dfsBottomUp(root, null);
}
private TreeNode dfsBottomUp(TreeNode p, TreeNode parent) {
if (p == null) return parent;
TreeNode root = dfsBottomUp(p.left, p);
p.left = (parent == null) ? parent : parent.right;
p.right = parent;
return root;
}