根据前序和后序遍历构造二叉树

/*
 * The problem is to construct a binary tree from given preorder and postorder traversals using Java Streams.
 * We will use a helper function that takes indices of the current subarray in preorder and postorder arrays.
 * Using the properties of preorder and postorder traversal, we will recursively construct the tree.
 */

import java.util.*;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { val = x; }
}

public class Solution {
    private int preIndex = 0;
    private int[] preorder;
    private int[] postorder;
    private Map<Integer, Integer> postIndexMap;

    public TreeNode constructFromPrePost(int[] preorder, int[] postorder) {
        this.preorder = preorder;
        this.postorder = postorder;
        this.postIndexMap = IntStream.range(0, postorder.length)
                                     .boxed()
                                     .collect(Collectors.toMap(i -> postorder[i], i -> i));
        return buildTree(0, postorder.length - 1);
    }

    private TreeNode buildTree(int left, int right) {
        if (left > right || preIndex >= preorder.length) {
            return null;
        }
        TreeNode root = new TreeNode(preorder[preIndex++]);
        if (left == right || preIndex >= preorder.length) {
            return root;
        }
        int postIndex = postIndexMap.get(preorder[preIndex]);
        root.left = buildTree(left, postIndex);
        root.right = buildTree(postIndex + 1, right - 1);
        return root;
    }
}