/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode buildTree(int[] preorder, int[] inorder) {

        return helper (0, 0, inorder.length - 1,  preorder, inorder);
    }

    private TreeNode helper(int preStart, int inStart, int inEnd, int[] preorder, int[] inorder) {

        //base condition
        if (preStart > preorder.length - 1 || inStart > inEnd)
            return null;

        //get the root node with curr preorder element
        TreeNode root = new TreeNode(preorder[preStart]);

        //get inIndex; finding preorder's element's index in inorder
        int inIndex = 0;

        for (int i = inStart; i <= inEnd; i++) {
            if(inorder[i] == root.val) {
                inIndex = i;
            }
        }
        //(next, left of inIndex is the end for left subtree)
        root.left = helper(preStart + 1, inStart, inIndex - 1, preorder, inorder);
        //(prestart + length of left subtree + 1)
        root.right = helper(preStart + inIndex - inStart + 1, inIndex + 1, inEnd, preorder, inorder);

        return root;
    }
    //T O(n) S O(n)
}