从 Rope 树中提取第 K 个字符

/*
题目思路:
使用Java Stream处理Rope树需要先将Rope树转换为字符串，然后使用Stream进行操作。
可以定义一个辅助函数，将Rope树中的所有字符拼接成一个完整的字符串，之后利用Stream获取第K个字符。
*/

import java.util.stream.IntStream;

// Rope树节点的定义
class RopeNode {
    String data;
    int weight;
    RopeNode left, right;
    
    RopeNode(String data) {
        this.data = data;
        this.weight = data.length();
        this.left = this.right = null;
    }
    
    RopeNode(int weight, RopeNode left, RopeNode right) {
        this.weight = weight;
        this.left = left;
        this.right = right;
        this.data = null;
    }
}

// Rope树类的定义
class RopeTree {
    RopeNode root;
    
    public RopeTree(RopeNode root) {
        this.root = root;
    }
    
    public char getKthCharacter(int k) {
        StringBuilder sb = new StringBuilder();
        buildString(root, sb);
        return IntStream.range(0, sb.length())
                        .mapToObj(sb::charAt)
                        .skip(k - 1)
                        .findFirst()
                        .orElseThrow(() -> new IllegalArgumentException("K exceeds the total number of characters in the Rope tree."));
    }
    
    private void buildString(RopeNode node, StringBuilder sb) {
        if (node == null) return;
        if (node.data != null) sb.append(node.data);
        buildString(node.left, sb);
        buildString(node.right, sb);
    }
}