Given the root of a binary tree, each node in the tree has a distinct value.

After deleting all nodes with a value in to_delete, we are left with a forest (a disjoint union of trees).

Return the roots of the trees in the remaining forest.  You may return the result in any order.

Example 1:

Input: root = [1,2,3,4,5,6,7], to_delete = [3,5]
Output: [[1,2,null,4],[6],[7]]


Constraints:

• The number of nodes in the given tree is at most 1000.
• Each node has a distinct value between 1 and 1000.
• to_delete.length <= 1000
• to_delete contains distinct values between 1 and 1000.

## Solution: Recursion / Post-order traversal

Recursively delete nodes on left subtree and right subtree and return the trimmed tree.
if the current node needs to be deleted, then its non-null children will be added to output array.

Time complexity: O(n)
Space complexity: O(|d| + h)

## Python3

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