Problem URL: Deepest leaves sum

We will run a BFS to get the sum of each level, then return the last level sum as result.

# 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 deepestLeavesSum(self, root: Optional[TreeNode]) -> int:
        q = collections.deque([root])
        while q:
            qLen = len(q)
            levelSum = 0
            for _ in range(qLen):
                node = q.pop()
                levelSum += node.val

                if node.left: q.appendleft(node.left)
                if node.right: q.appendleft(node.right)

        return levelSum

Time Complexity: O(n)
Space Complexity: O(n)