前序遍历

 public List<Integer> preorderTraversal(TreeNode root) {
        ArrayList<Integer> list = new ArrayList<Integer>();
        Stack<TreeNode> stack=new Stack<TreeNode>();
        TreeNode p=root;
        while(!stack.isEmpty()||p!=null){
            if(p!=null){
                stack.push(p);
                list.add(p.val); // Add before going to children
                p=p.left;
            }else{
                TreeNode node=stack.pop();
                p=node.right;
            }
        }
		return list;
    }　　

中序遍历

 public List<Integer> inorderTraversal(TreeNode root) {
        ArrayList<Integer> list = new ArrayList<Integer>();
        Stack<TreeNode> stack=new Stack<TreeNode>();
        TreeNode p=root;
        while(!stack.isEmpty()||p!=null){
            if(p!=null){
                stack.push(p);
                p=p.left;
            }else{
                TreeNode node=stack.pop();
                list.add(node.val);// Add after all left children
                p=node.right;
            }
        }
		return list;
    }

后序遍历

public List<Integer> postorderTraversal(TreeNode root) {
        LinkedList<Integer> list = new LinkedList<Integer>();
        Stack<TreeNode> stack=new Stack<TreeNode>();
        TreeNode p=root;
        while(!stack.isEmpty()||p!=null){
            if(p!=null){
                stack.push(p);
                list.addFirst(p.val);// Reverse the process of preorder
                p=p.right;
            }else{
                TreeNode node=stack.pop();
                p=node.left;
            }
        }
		return list;
    }