LC144. Binary Tree Preorder Traversal

Approach 1: Recursive

# 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: Optional[TreeNode]) -> List[int]:
        result = []
 
        def traverse(node):
            if node is None:
                return
            
            result.append(node.val)
            traverse(node.left)
            traverse(node.right)
 
        traverse(root)
 
        return result

Complexity

Time: $O(n)$
Space: $O(n)$

Approach 2: Iterative

# 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: Optional[TreeNode]) -> List[int]:        
        stack = [root]
        result = []
 
        while stack:
            node = stack.pop()
 
            if node is None:
                continue
 
            result.append(node.val)
            stack.append(node.right)
            stack.append(node.left)
 
        return result

Complexity

Time: $O(n)$
Space: $O(n)$