Kth largest sum in a binary tree

May 21, 2023

heap tree

Problem URL: Kth largest sum in a binary tree

We will traverse the whole tree with BFS and store each levels sum in a list. Then, we will use a heap to get the k-th largest sum. We will also check if the tree doesn't have enough levels to reach k, then we return -1.

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right

class Solution:
    def kthLargestLevelSum(self, root: Optional[TreeNode], k: int) -> int:
        levels = []
        q = collections.deque([root])
        while q:
            qLen, levelSum = len(q), 0
            for _ in range(qLen):
                node = q.popleft()
                levelSum += node.val
                if node.left: q.append(node.left)
                if node.right: q.append(node.right)

        if len(levels) < k:
            return -1

        return heapq.nlargest(k, levels)[-1]

Time complexity: O(nlog(k)) where n is the number of nodes in the tree.
Space complexity: O(n) where n is the number of nodes in the tree.