MostFrequentSubtreeSum.java

package tree;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * Created by gouthamvidyapradhan on 27/03/2017.
 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.
 */
public class MostFrequentSubtreeSum
{
    static class TreeNode
    {
        int val;
        TreeNode left;
        TreeNode right;
        TreeNode(int x) { val = x; }
    }

    private Map<Integer, List<Integer>> fList = new HashMap<>();
    private Map<Integer, Integer> fCount = new HashMap<>();
    /**
     * Main method
     * @param args
     * @throws Exception
     */
    public static void main(String[] args) throws Exception
    {
        MostFrequentSubtreeSum mfs = new MostFrequentSubtreeSum();
        TreeNode node = new TreeNode(5);
        //node.left = new TreeNode(2);
        //node.right = new TreeNode(-5);
        int[] result = mfs.findFrequentTreeSum(node);
    }

    public int[] findFrequentTreeSum(TreeNode root)
    {
        int[] resArr = new int[0];
        if(root == null) return resArr;
        postOrder(root);
        for (Map.Entry<Integer, Integer> entry : fCount.entrySet())
        {
            int frequency = entry.getValue();
            List<Integer> list = fList.get(frequency);
            if(list == null)
                list = new ArrayList<>();
            list.add(entry.getKey());
            fList.put(frequency, list);
        }
        int max = Integer.MIN_VALUE;
        List<Integer> result;
        for(int key : fList.keySet())
            max = Math.max(max, key);
        result = fList.get(max);
        resArr = new int[result.size()];
        for(int i = 0, l = resArr.length; i < l; i ++)
            resArr[i] = result.get(i);
        return resArr;
    }

    private int postOrder(TreeNode root)
    {
        if(root == null) return 0;
        int sum = postOrder(root.left) + postOrder(root.right) + root.val;
        Integer fSum = fCount.get(sum);
        if(fSum == null)
            fCount.put(sum, 1);
        else fCount.put(sum, fSum + 1);
        return sum;
    }

}