[LeetCode]144. Binary Tree Preorder Traversal二叉树前序遍历

/*
    考察基本功的一道题，迭代实现二叉树前序遍历
     */
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if (root==null)
            return res;
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty())
        {
            TreeNode cur = stack.pop();
            res.add(cur.val);
            if (cur.right!=null)
                stack.push(cur.right);
            if (cur.left!=null)
                stack.push(cur.left);
        }
        return res;
    }