144. Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3},

   1
    
     2
    /
   3

return [1,2,3].

Note: Recursive solution is trivial, could you do it iteratively?

与94题很类似，94题是inorder。

用stack，先将左子树push进stack并存进结果的list里。然后到null的时候指向右子数。

    public IList<int> PreorderTraversal(TreeNode root) {
        var res = new List<int>();
        var stack = new Stack<TreeNode>();
        
        while(root != null || stack.Count() > 0)
        {
            while(root != null)
            {
                res.Add(root.val);
                stack.Push(root);
                root = root.left;
            }
            TreeNode a  = stack.Pop();
            root = a.right;
        }
        return res;
    }