Solution.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    private Map<Integer, Integer> counter;
    private int mx;

    public int[] findFrequentTreeSum(TreeNode root) {
        counter = new HashMap<>();
        mx = Integer.MIN_VALUE;
        dfs(root);
        List<Integer> res = new ArrayList<>();
        for (Map.Entry<Integer, Integer> entry : counter.entrySet()) {
            if (entry.getValue() == mx) {
                res.add(entry.getKey());
            }
        }
        int[] ans = new int[res.size()];
        for (int i = 0; i < res.size(); ++i) {
            ans[i] = res.get(i);
        }
        return ans;
    }

    private int dfs(TreeNode root) {
        if (root == null) {
            return 0;
        }
        int s = root.val + dfs(root.left) + dfs(root.right);
        counter.put(s, counter.getOrDefault(s, 0) + 1);
        mx = Math.max(mx, counter.get(s));
        return s;
    }
}