Sum of Roots in Binary Search Tree

Binary Search TreeDepth-First SearchTree

Given a Binary Search Tree (BST), calculate the sum of the values of all the root nodes from the individual subtrees. A root node of a subtree is defined as the node that does not have a parent node within its subtree.

Example 1

Input

[
  [
    10,
    5,
    15,
    3,
    7,
    null,
    18
  ]
]

Output

Explanation. This tree contains only one subtree which is the tree itself. The root node is 10.

Example 2

Input

Output

Explanation. The tree has following roots of subtrees - 10, 15, 5, 3. Sum = 10 + 15 + 5 + 3 = 33.

Follow-up: Can you optimize your solution to handle large trees efficiently?

Constraints:

The given binary tree will have at least one node.

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