好叶子节点对的数量

// 思路：
// 1. 使用递归的方式找到所有叶子节点，并计算它们到根节点的路径长度。
// 2. 将所有叶子节点的距离存储在一个列表中，使用流操作来过滤和计数满足条件的叶子节点对。

import java.util.*;
import java.util.stream.*;

class Solution {
    public int countPairs(TreeNode root, int distance) {
        List<Integer> leafDistances = new ArrayList<>();
        calculateLeafDistances(root, 0, leafDistances);
        return (int) IntStream.range(0, leafDistances.size()).boxed()
            .flatMap(i -> IntStream.range(i + 1, leafDistances.size())
            .mapToObj(j -> Math.abs(leafDistances.get(i) - leafDistances.get(j))))
            .filter(d -> d <= distance)
            .count();
    }

    private void calculateLeafDistances(TreeNode node, int currentDistance, List<Integer> leafDistances) {
        if (node == null) return;
        if (node.left == null && node.right == null) {
            leafDistances.add(currentDistance);
            return;
        }
        calculateLeafDistances(node.left, currentDistance + 1, leafDistances);
        calculateLeafDistances(node.right, currentDistance + 1, leafDistances);
    }
}