508. Most Frequent Subtree Sum

题目描述

题目链接

https://leetcode.com/problems/most-frequent-subtree-sum/

方法思路

class Solution {
    //Runtime: 7 ms, faster than 86.00%
    //Memory Usage: 40.6 MB, less than 39.71%
    //键值对的映射，取sum为key,frequency为value
    Map<Integer, Integer> map;
    int frequency;
    public int[] findFrequentTreeSum(TreeNode root) {
        map = new HashMap<>();
        frequency = 1;
        sum_Helper(root);
        List<Integer> list = new LinkedList<>();
        for(int x : map.keySet()){
            if(map.get(x) == frequency)
                list.add(x);
        }
        int[] ans = new int[list.size()];
        int i = 0;
        for(int x : list)
            ans[i++] = x;
        return ans;
    }
    public int sum_Helper(TreeNode root){
        if(root == null) return 0;
        int left = sum_Helper(root.left);
        int right = sum_Helper(root.right);
        int sum = root.val + left + right;
        map.put(sum, map.getOrDefault(sum, 0) + 1);
        if(frequency < map.get(sum)) 
            frequency = map.get(sum);
        return sum;
    }
}