#889 - Construct Binary Tree from Preorder and Postorder Traversal

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Problem Description

Given two integer arrays, preorder and postorder where preorder is the preorder traversal of a binary tree of distinct values and postorder is the postorder traversal of the same tree, reconstruct and return the binary tree.

If there exist multiple answers, you can return any of them.

Solution

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {number[]} preorder
 * @param {number[]} postorder
 * @return {TreeNode}
 */
var constructFromPrePost = function(preorder, postorder) {
  if (!preorder.length) return null;

  const root = new TreeNode(preorder[0]);
  if (preorder.length === 1) return root;

  const leftSize = postorder.indexOf(preorder[1]) + 1;
  root.left = constructFromPrePost(
    preorder.slice(1, leftSize + 1),
    postorder.slice(0, leftSize)
  );
  root.right = constructFromPrePost(
    preorder.slice(leftSize + 1),
    postorder.slice(leftSize, -1)
  );

  return root;
};