144. Binary Tree Preorder Traversal - LeetCode Solution

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def preorderTraversal(self, root: TreeNode) -> List[int]:
        res = []
        stack = [root]                                      
        while(stack):                           
            currNode = stack.pop()
            if(currNode):
                res.append(currNode.val)                   
                if(currNode.right):
                    stack.append(currNode.right)           
                if(currNode.left):                     
                    stack.append(currNode.left)             
                    
        return res