[LeetCode156] Binary Tree Upside Down

Given a binary tree where all the right nodes are either leaf nodes with a sibling (a left node that shares the same parent node) or empty, flip it upside down and turn it into a tree where the original right nodes turned into left leaf nodes. Return the new root.

For example:
Given a binary tree {1,2,3,4,5},
1
/ \
2 3
/ \
4 5

return the root of the binary tree [4,5,2,#,#,3,1].
4
/ \
5 2
  / \

 3 1

	public TreeNode UpsideDownBinaryTree(TreeNode root) {
		if (root == null)
			return null;
		TreeNode parent = root, left = root.left, right = root.right;
		if (left != null) {
			TreeNode ret = UpsideDownBinaryTree(left);
			left.left = right;
			left.right = parent;
			return ret;
		}
		return root;
	}