창고는 두 갈래 나무가 반복적으로 흐르도록 실현한다

전순, 중순, 후순 통용

public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        Deque<TreeNode> stack = new LinkedList<>();
        if(root != null){
            stack.push(root);
        }
        while(!stack.isEmpty()){
            TreeNode cur = stack.pop();
            if(cur != null){
            	// ， ， ，
            	// ， 
                stack.push(cur);
                //cur ， ， （ ）
                stack.push(null);
                // ， cur，left，righ 
                if(cur.right != null)
                    stack.push(cur.right);
                if(cur.left != null)
                    stack.push(cur.left);    
            }else{
                res.add(stack.pop().val);
            }
        }
        return res;
    }

일반 교체

// 
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new LinkedList<>();
        if (root == null) return res;
        Deque<TreeNode> stack = new LinkedList<>();
        TreeNode cur = root;
        while (cur != null || !stack.isEmpty()) {
            if (cur != null) {
                res.add(cur.val);
                stack.push(cur.right);
                cur = cur.left;
            } else cur = stack.pop();
        }
        return res;
    }

    // 
    public List<Integer> inorderTraversal(TreeNode root) {
        Deque<TreeNode> stack = new LinkedList<>();
        List<Integer> list = new LinkedList<>();
        if (root == null) return list;
        TreeNode cur = root;
        while (cur != null || !stack.isEmpty()) {
            if (cur != null) {
                stack.push(cur);
                cur = cur.left;
            } else {
                cur = stack.pop();
                list.add(cur.val);
                cur = cur.right;
            }
        }
        return list;
    }

    // 
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new LinkedList<>();
        Deque<TreeNode> stack = new LinkedList<>();
        TreeNode cur = root, pre = null;
        while (cur != null || !stack.isEmpty()) {
            if (cur != null) {
                stack.push(cur);
                cur = cur.left;
            } else {
                cur = stack.peek();
                //  
                if (cur.right == null || cur.right == pre) {
                    res.add(cur.val);
                    stack.pop();
                    pre = cur;
                    cur = null;
                } else cur = cur.right;
            }
        }
        return res;
    }