Solution.java

import java.util.*;

public class Solution
{
    private int maxCount = Integer.MIN_VALUE;

    public int[] findFrequentTreeSum(TreeNode root)
    {
        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        getSums(root, map);
        List<Integer> list = new ArrayList<>();
        map.forEach((k, v) ->
        {
            if (v == maxCount)
            {
                list.add(k);
            }
        });
        int[] result = new int[list.size()];
        for (int i = 0; i < list.size(); i++)
        {
            result[i] = list.get(i);
        }
        return result;
    }

    private int getSums(TreeNode root, Map<Integer, Integer> map)
    {
        int sum = root == null ? 0 : root.val + getSums(root.left, map) + getSums(root.right, map);
        if (root != null)
        {
            map.put(sum, map.getOrDefault(sum, 0) + 1);
            maxCount = Math.max(maxCount, map.get(sum));
        }
        return sum;
    }
}