The Runtime Theory
mediumLeetCode#recursion#tree-construction

Construct Binary Tree from Traversal

Reconstruct a unique-value binary tree from its preorder and inorder traversals.

The Runtime Theory Team25 min read
Solve it

Solving happens on the judge — come back and mark it done

Sample cases

inpreorder=[3,9,20,15,7], inorder=[9,3,15,20,7]

out[3,9,20,null,null,15,7]

inpreorder=[-1], inorder=[-1]

out[-1]

inpreorder=[], inorder=[]

out[]

Reconstruct a unique-value binary tree from its preorder and inorder traversals.

A good solution should

  • State the invariant or decision rule that makes the approach correct.
  • Explain the time and space complexity in terms of the input sizes and required output.
  • Handle empty, minimal, duplicate, and boundary-shaped inputs covered by the contract.

Reasoning prompt

Use preorder to select the root and inorder to divide left from right. Avoid linear searches at every recursion by indexing positions.

Use the linked judge for the canonical problem. The examples above are compact TRT checks; write additional cases before submitting, especially for the boundary that tends to break your chosen invariant.

More practice in this topic

easy · treesBalanced Binary Treemedium · treesBinary Tree Level Order Traversalmedium · treesBinary Tree Right Side View

One dispatch a week

The trace behind each problem, the tradeoff that explains it, and one technical dispatch per week — no noise.

One technical dispatch per week. No noise.

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