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10. Binary Tree Inorder Traversal

easyAsked at CircleCI

Return the in-order 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 (left, node, right) traversal of its node values. Solve it both recursively and iteratively.

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

Walk left, visit node, walk right.

Time
O(n)
Space
O(h)
const out = [];
function 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 lefts onto a stack, pop and visit, then descend the right subtree.

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

Tradeoff:

CircleCI-specific tips

CircleCI interviewers like to see the iterative version because their job-graph traversal code avoids deep recursion on user-defined workflows.

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Output

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