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Leetcode
Medium
Trees
Recursion
Stacks
Binary Tree Inorder Traversal

Return the inorder traversal of a binary tree: visit left subtree, then node, then right subtree.

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Problem Statement

Problem

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

In inorder traversal, you visit:

  1. the left subtree
  2. the current node
  3. the right subtree

You may solve this using either recursion or an explicit stack.

Notes

  • The tree can be empty.
  • Each node value should appear exactly once in the returned order.

Input Format

  • A single binary tree root node is provided as input.
  • The tree is represented in the usual linked-node form with val, left, and right references.

Output Format

  • Return an array of integers containing the inorder traversal of the tree.

Constraints

  • The number of nodes is non-negative.
  • Node values can be any integers.
  • Time complexity should be linear in the number of nodes.
  • Auxiliary space depends on the traversal method used.
Examples
Sample cases returned by the problem API.

Example 1

Input

root = [1,null,2,3]

Output

[1,3,2]

Explanation

Traverse left of 1 (none), then 1, then inorder of 2 which yields 3 before 2.

Example 2

Input

root = [1,2,3,4,5,null,6]

Output

[4,2,5,1,3,6]

Explanation

Visit the entire left subtree of 1 first, then 1, then the right subtree.

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