后序遍历二叉树

递归方法和之前的几乎一致，不再赘述。

考虑非递归方法：
后序遍历顺序是：左，右，根
即左孩子和右孩子都遍历输出了之后，才遍历子树的根节点。

public ArrayList<Integer> postorderTraversal(TreeNode root) {

        ArrayList<Integer> lst = new ArrayList<Integer>();

        if(root == null)
            return lst; 

        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(root);

        TreeNode prev = null;
        while(!stack.empty()){
            TreeNode curr = stack.peek();

            // go down the tree.
            //check if current node is leaf, if so, process it and pop stack,
            //otherwise, keep going down
            if(prev == null || prev.left == curr || prev.right == curr){
                //prev == null is the situation for the root node
                if(curr.left != null){
                    stack.push(curr.left);
                }else if(curr.right != null){
                    stack.push(curr.right);
                }else{
                    stack.pop();
                    lst.add(curr.val);
                }

            //go up the tree from left node    
            //need to check if there is a right child
            //if yes, push it to stack
            //otherwise, process parent and pop stack
            }else if(curr.left == prev){
                if(curr.right != null){
                    stack.push(curr.right);
                }else{
                    stack.pop();
                    lst.add(curr.val);
                }

            //go up the tree from right node 
            //after coming back from right node, process parent node and pop stack. 
            }else if(curr.right == prev){
                stack.pop();
                lst.add(curr.val);
            }

            prev = curr;
        }

        return lst;
    }