# x 105. Construct Binary Tree from Preorder and Inorder Traversal

[preorder-inorder-postorder](https://algorithm.prettylog.com/overview/2.-data-structures/tree/preorder-inorder-postorder "mention")

{% embed url="<https://leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/description/>" %}

<https://8iggy.tistory.com/112>

> time: O(n)

> space: O(n)

```jsx
/**
 * 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[]} inorder
 * @return {TreeNode}
 */

let preorderIdx = 0;
let inroderMap;
var buildTree = function (preorder, inorder) {
    preorderIdx = 0;
    inorderMap = inorder.reduce((acc, curr, i) => (acc[curr] = i, acc), {});
    return construct(preorder, 0, preorder.length - 1);
};

function construct(preorder, left, right) {
    if (preorderIdx >= preorder.length) {
        return null;
    }

    if (left > right) {
        return null;
    }

    const value = preorder[preorderIdx++];
    const node = new TreeNode(value);

    const inorderIdx = inorderMap[value];

    node.left = construct(preorder, left, inorderIdx - 1);
    node.right = construct(preorder, inorderIdx + 1, right);

    return node;
}
```