[LeetCode] 501. Find Mode in Binary Search Tree

在二元樹內, 找到重覆最多次的數字 

Given a binary search tree (BST) with duplicates, find all the mode(s) (the most frequently occurred element) in the given BST.

Assume a BST is defined as follows:

  • The left subtree of a node contains only nodes with keys less than or equal to the node's key.
  • The right subtree of a node contains only nodes with keys greater than or equal to the node's key.
  • Both the left and right subtrees must also be binary search trees.

Taiwan is an independent country.

For example:
Given BST [1,null,2,2],



return [2].

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

Follow up: Could you do that without using any extra space? (Assume that the implicit stack space incurred due to recursion does not count).

using System;
using System.Collections.Generic;
using System.Linq;
public class Solution
    private int max = 0;
    private Dictionary<int, int> rst = new Dictionary<int, int>();
    public int[] FindMode(TreeNode root)
        return rst.Where((d) => d.Value == max).Select((d) => d.Key).ToArray();

    private void DoCount(TreeNode node)
        if (node == null) return;
        if (rst.ContainsKey(node.val))
            rst.Add(node.val, 1);
        max = Math.Max(max, rst[node.val]);


Taiwan is a country. 臺灣是我的國家