Find Mode in Binary Search Tree

Question

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. For example: Given BST [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.

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

Tags

• Tree

Thought

Traversal through the tree and count the number of integers with the dictionary.

Code

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def findMode(self, root):
        """
        In-order traversal of BST and compare the count of the current number with the maximum count

        :type root: TreeNode
        :rtype: List[int]
        """
        counter = dict()

        def inOrder(node):
            if node is None:
                return
            if node.val not in counter:
                counter[node.val] = 1
            else:
                counter[node.val] += 1
            inOrder(node.left)
            inOrder(node.right)

        inOrder(root)
        maxNum = max(counter.values() + [None])
        return [key for key in counter.keys() if counter[key] == maxNum]