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10. Binary Tree Inorder Traversal

easyAsked at Wix

Return the inorder traversal of a binary tree's values; Wix uses the pattern to walk a page's nested component tree in render order.

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

Problem

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

Constraints

  • 0 <= number of 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, visit, recurse right.

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

Tradeoff:

2. Iterative stack

Use an explicit stack to walk left, then pop and go right.

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

Tradeoff:

Wix-specific tips

Wix often follows up by asking how you'd avoid stack overflow on deep component trees — mention the iterative approach for builder hierarchies that nest 50+ deep.

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Output

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