https://leetcode.com/contest/leetcode-weekly-contest-17/problems/find-mode-in-binary-tree/

Find_Mode_in_Binary_Tree.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    
    public static Map<Integer, Integer> aMap = new HashMap<Integer, Integer>();

	public static int[] findMode(TreeNode root) {
	    aMap.clear();
		if (root == null) {
			return null;
		}
		getMode(root);
		int idx = 0;
		int max = 0;
		for (int i : aMap.keySet()) {
			if (aMap.get(i) >= max) {
				max = aMap.get(i);
			}
		}
		List<Integer> al = new ArrayList<Integer>();
		for (int i : aMap.keySet()) {
			if (aMap.get(i) == max) {
				al.add(i);
			}
		}
		int[] ansi = new int[al.size()];
		for (int i = 0; i < al.size(); i++) {
			ansi[i] = al.get(i);
		}
		return ansi;
	}

	private static void getMode(TreeNode root) {
		if (root == null) {
			return;
		}
		if (aMap.containsKey(root.val)) {
			int c = aMap.get(root.val);
			aMap.put(root.val, ++c);
		} else {
			aMap.put(root.val, 1);
		}
		getMode(root.right);
		getMode(root.left);
	}
    
}

Given a binary tree with duplicates. You have to find all the mode(s) in given binary tree.

For example:
Given binary tree [1,null,2,2],
1

2
/
2
return [2].

Note: If a tree has more than one mode, you can return them in any order.