Reconstruction of binary tree

题目：
输入某二叉树的前序遍历和中序遍历的结果,请构建该二叉树并返回其根节点.
假设输入的前序遍历和中序遍历的结果中都不含重复的数字.
示例一：
Input: preorder = [3,9,20,15,7], inorder = [9,3,15,20,7]
Output: [3,9,20,null,null,15,7]
分析：
find recursion

代码：

import java.util.HashMap;
import java.util.Map;

public class BuildTree {
    
    // Use the hash function to easily get the subscript
    Map<Integer, Integer> map = new HashMap<>();
    public TreeNode buildTree(int[] preorder, int[] inorder) {
    
        if (preorder == null || preorder.length == 0){
    
            return null;
        }

        for (int i = 0; i < inorder.length; i++) {
    
            map.put(inorder[i],i);
        }
        TreeNode root = f(preorder,0,preorder.length-1,inorder,0,inorder.length-1);
        return root;

    }

    private TreeNode f(int[] preorder, int l1, int r1, int[] inorder, int l2, int r2) {
    
        if (l1 > r1 || l2 > r2){
    
            return null;
        }
        TreeNode root = new TreeNode(preorder[l1]);
        //Get the root node subscript
        int i = map.get(preorder[l1]);
        root.left = f(preorder,l1+1,l1+i-l2,inorder,l2,i-1);
        root.right = f(preorder,l1+i-l2+1,r1,inorder,i+1,r2);
        return root;
    }
}