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9. Binary Tree Inorder Traversal

easyAsked at Electronic Arts

Return the inorder traversal of a binary tree's node values using recursion or an explicit stack.

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

Problem

Given the root of a binary tree, return the inorder traversal of its node values as a list.

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 node, recurse right.

Time
O(n)
Space
O(h)
const out=[];
const dfs=n=>{if(!n) return; dfs(n.left); out.push(n.val); dfs(n.right);};
dfs(root);
return out;

Tradeoff:

2. Iterative with stack

Push all left children to a stack, pop and visit, then move to the right subtree.

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:

Electronic Arts-specific tips

EA expects you to know both recursive and iterative variants — gameplay scene graphs at EA are deep enough that recursion-depth concerns are real.

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Output

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