出现次数最多的子树元素和

/*
 * 题目思路：
 * 1. 使用后序遍历计算每个节点的子树和，使用Stream API对结果进行处理。
 * 2. 通过计算每个子树和的出现频率，找出出现次数最多的子树和。
 * 3. 使用Collectors.groupingBy和Collectors.counting来实现统计。
 */
 
import java.util.*;
import java.util.stream.*;
 
class TreeNode {
    int val;
    TreeNode left, right;
    TreeNode(int x) { val = x; }
}
 
public class Solution {
    public int[] findFrequentTreeSum(TreeNode root) {
        Map<Integer, Long> sumCount = new HashMap<>();
        calculateSubtreeSum(root, sumCount);
        long maxCount = sumCount.values().stream().max(Long::compare).orElse(0L);
        return sumCount.entrySet().stream()
                        .filter(entry -> entry.getValue() == maxCount)
                        .map(Map.Entry::getKey)
                        .mapToInt(i -> i)
                        .toArray();
    }
 
    private int calculateSubtreeSum(TreeNode node, Map<Integer, Long> sumCount) {
        if (node == null) return 0;
        int leftSum = calculateSubtreeSum(node.left, sumCount);
        int rightSum = calculateSubtreeSum(node.right, sumCount);
        int totalSum = node.val + leftSum + rightSum;
        sumCount.put(totalSum, sumCount.getOrDefault(totalSum, 0L) + 1);
        return totalSum;
    }
}