Binary Tree Inorder Traversal

Published: Sep 30, 2022

Introduction

This is a basic inorder traversal problem. Just visit all nodes in the binary tree by the inorder manner.

Problem Description

Given the root of a binary tree, return the inorder traversal of its nodes’ values.

Constraints:

• The number of nodes in the tree is in the range [0, 100].
• -100 <= Node.val <= 100

https://leetcode.com/problems/binary-tree-inorder-traversal/

Examples

Example 1
Input: root = [1,null,2,3]
Output: [1,3,2]
Explanation:
1
 \
  2
 /
3

Example 2
Input: root = []
Output: []

Example 3
Input: root = [1]
Output: [1]

Analysis

The tree traversal takes the depth-first search (DFS) style. The helper function does the DFS and saves values between going left and right.

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 BinaryTreeInorderTraversal:
    def inorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        def helper(root, result):
            if not root:
                return result
            result = helper(root.left, result)
            result.append(root.val)
            result = helper(root.right, result)
            return result
        return helper(root, [])

Complexities

• Time: O(n)
• Space: O(n)