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9. Binary Tree Inorder Traversal

easyAsked at Zoom

Return the inorder traversal of a binary tree's node values.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

Given the root of a binary tree, return the inorder traversal of its nodes' values. Visit left subtree, then root, then right subtree.

Constraints

  • 0 <= nodes <= 100
  • -100 <= Node.val <= 100

Examples

Example 1

Input
root=[1,null,2,3]
Output
[1,3,2]

Example 2

Input
root=[]
Output
[]

Approaches

1. Recursive

Recurse left, push root, recurse right.

Time
O(n)
Space
O(h)
function inorder(r,out=[]){ if(!r) return out; inorder(r.left,out); out.push(r.val); inorder(r.right,out); return out;}

Tradeoff:

2. Iterative with stack

Push left descendants on a stack; pop, record, then move right.

Time
O(n)
Space
O(h)
function inorderTraversal(root) {
  const out = [], stack = [];
  let cur = root;
  while (cur || stack.length) {
    while (cur) { stack.push(cur); cur = cur.left; }
    cur = stack.pop();
    out.push(cur.val);
    cur = cur.right;
  }
  return out;
}

Tradeoff:

Zoom-specific tips

Zoom's UI tree of meeting rooms / breakout subrooms is traversed inorder for the participant-roster panel — show the iterative version to prove stack-depth safety.

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Output

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