翻转二叉树以匹配先序遍历

import java.util.*;
import java.util.function.Function;
import java.util.stream.Collectors;

// This class defines the structure of a binary tree node.
class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { val = x; }
}

// This class contains the solution to the problem using Java Streams.
class Solution {
    private List<Integer> result = new ArrayList<>();
    private int index = 0;

    // Method to flip the binary tree to match the given voyage using streams.
    public List<Integer> flipMatchVoyage(TreeNode root, int[] voyage) {
        if (!dfs(root, voyage)) {
            return Collections.singletonList(-1);
        }
        return result;
    }

    // Helper method to perform depth-first search and flip nodes if necessary using streams.
    private boolean dfs(TreeNode node, int[] voyage) {
        if (node == null) return true;
        if (node.val != voyage[index++]) return false;
        if (node.left != null && node.left.val != voyage[index]) {
            result.add(node.val);
            return dfs(node.right, voyage) && dfs(node.left, voyage);
        }
        return dfs(node.left, voyage) && dfs(node.right, voyage);
    }
}