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The problem description is pretty straightforward. We have to return all the nodes’ values following the preorder traversal algorithm.
I can assume that you have a basic understanding of Tree data structure.
Mainly, there are four types of traversing algorithms in Binary Tree. Preorder traversal is one of them.
These algorithms follow different rules to visit nodes’ in a Binary Tree. In this case (preorder), it follows the rule ‘root->left->right‘ when visiting a Binary Tree.
We can solve this problem in both ways, iterative and recursive.
Let’s see both solutions.
class Solution: def preorderTraversal(self, root: TreeNode) -> List[int]: if not root: return  result =  stack = [root] while stack: node = stack.pop() result.append(node.val) if node.right: stack.append(node.right) if node.left: stack.append(node.left) return result
class Solution: def preorderTraversal(self, root: TreeNode) -> List[int]: result =  self.helper(root, result) return result def helper(self, root, result): if root is None: return result.append(root.val) self.helper(root.left, result) self.helper(root.right, result)