把一棵二叉树压平

/*
采用后序遍历的方法来实现
*/
private TreeNode prev = null;
public void flatten(TreeNode root){
	if(root == null){
		return;
	}
	flatten(root.right);
	flatten(root.left);
	root.right = prev;
	root.left = null;
	prev = root;
}