private static LeetCode114TreeNode Helper(LeetCode114TreeNode root)
{
if (root == null)
{
return(null);
}
if (root.IsLeaf())
{
return(root);
}
var left = Helper(root.left);
var right = Helper(root.right);
if (left != null)
{
left.right = root.right;
root.right = root.left;
root.left = null;
}
return(right == null ? left : right);
}
