145. Binary Tree Postorder Traversal

    /*
     * 145. Binary Tree Postorder Traversal 
     * 2015.11.21 by Mingyang
     * 左右中，反过来就是中左右，所以就相当于inorder反过来，稍稍改进一下就好
     */
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<Integer>();
        if (root == null)
            return list;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode p = root;
        while (!stack.isEmpty() || p != null) {
            if (p != null) {
                stack.push(p);
                list.add(p.val);
                p = p.right;
            } else {
                TreeNode t = stack.pop();
                p = t.left;
            }
        }
        Collections.reverse(list);
        return list;
    }
    public List<Integer> postorderTraversal1(TreeNode root) {
        List<Integer> res=new ArrayList<Integer>();
        dfs(root,res);
        return res;
    }
    public void dfs1(TreeNode root,List<Integer> res){
        if(root==null)
         return;
        dfs1(root.left,res);
        dfs1(root.right,res);
        res.add(root.val);
    }