Leetcode -- 508. Most Frequent Subtree Sum(DFS)

508. Most Frequent Subtree Sum(DFS)
Medium
https://leetcode.com/problems/most-frequent-subtree-sum/?tab=Description

Given the root of a tree, you are asked to find the most frequent subtree sum. The subtree sum of a node is defined as the sum of all the node values formed by the subtree rooted at that node (including the node itself). So what is the most frequent subtree sum value? If there is a tie, return all the values with the highest frequency in any order.

Examples 1
Input:

  5
 /  \
2   -3

return [2, -3, 4], since all the values happen only once, return all of them in any order.

Examples 2
Input:

  5
 /  \
2   -5

return [2], since 2 happens twice, however -5 only occur once.

Note: You may assume the sum of values in any subtree is in the range of 32-bit signed integer.

思路： Use HashMap + PostOrder DFS tree traversal.
Complexity: Time: O(N) - tree traversal, Space: O(N) - map

public class Solution {
    Map<Integer, Integer> map = new HashMap();
    int maxCount = 0;
    public int[] findFrequentTreeSum(TreeNode root) {
        //use DFS
        List<Integer> tmp = new ArrayList();
        helper(root);
        for(Integer k:map.keySet()){
            if(map.get(k) == maxCount) tmp.add(k);
        }
        int[] res = new int[tmp.size()];
        int i = 0;
        for(Integer v:tmp){
            res[i++] = v;
        }
        return res;
    }
    
    public int helper(TreeNode root){
        if(root == null) return 0;
        int sum = root.val;
        sum += helper(root.left);
        sum += helper(root.right);
        if(!map.containsKey(sum)) map.put(sum, 0);
        map.put(sum, map.get(sum) + 1);
        maxCount = Math.max(maxCount, map.get(sum));
        return sum;
    }
}