2019.4.2

2019.4.2

public void postorderUsingStack(){
        postorderUsingStack(root);
    }

    private void postorderUsingStack(TreeNode<T> root){
        if(root == null)
            return;

        Stack<TreeNode<T>> stack = new Stack<>();

        TreeNode<T> current = root;
        TreeNode<T> pre = null;

        while(current != null){
            stack.push(current);
            current = current.left;
        }
        while(!stack.empty()){
            current = stack.pop();
            if(current.right != null && current.right != pre){
                stack.push(current);
                current = current.right;
                while (current != null){
                    stack.push(current);
                    current = current.left;
                }
            }
            else{
                System.out.print(current.element+" ");
                pre = current;
            }
        }
    }