144 Binary Tree Preorder Traversal

This is a fundamental tree problem that requires us to collect the preorder traversal of a tree in a list and return it. A preorder traversal is a tree/graph traversal where you visit the first node (root in a case of a tree), and then it’s children.

Code:

Python

# 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]:
        res = []
        def dfs(root: Optional[TreeNode]):
            if root == None: return None
 
            res.append(root.val)
 
            dfs(root.left)
            dfs(root.right)
 
            return None
 
        dfs(root)
 
        return res

Big O Analysis

• Runtime The runtime complexity here is O(N) as since we would be visiting all the nodes in the tree.
• Memory The memory usage is O(1) since we are not using any extra datastructure (usually the returning result array does not count, if it counts, then it’s O(N)).

— A

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