Binary Tree Upside Down

//TODO

• https://leetcode.com/problems/binary-tree-upside-down/

public class Solution {
    private TreeNode prev;
    public TreeNode UpsideDownBinaryTree(TreeNode root) {
        TreeNode p = root, parent = null, parentRight = null;
        while (p != null) {
            TreeNode left = p.left;
            p.left = parentRight;
            parentRight = p.right;
            p.right = parent;
            parent = p;
            p = left;
        }
        return parent;
    }

}