Tree traversals

class Solution:
    def inorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        if not root:
            return []
        
        res = []
        stack = []
        
        while stack or root:
            while root:
                stack.append(root)
                root = root.left
                
            root = stack.pop()
            res.append(root.val)
            root = root.right
        return res

class Solution:
    def preorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        if not root:
            return []
        
        res = []
        
        stack = [root]
        
        while stack:
            root = stack.pop()
            
            if root:
                res.append(root.val)
                if root.right:
                    stack.append(root.right)
                
                if root.left:
                    stack.append(root.left)
                    
        return res