Jobs

Interviews

Discussions

Average of Levels in Binary Tree

Easy

Amazon

3 views

Topics:

Trees

Given the root of a binary tree, return the average value of the nodes on each level in the form of an array. Answers within 10^-5 of the actual answer will be accepted.

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: [3.00000,14.50000,11.00000]
Explanation: The average value of nodes on level 0 is 3, on level 1 is 14.5, and on level 2 is 11.
Hence return [3, 14.5, 11].

Example 2:

Input: root = [3,9,20,15,7]
Output: [3.00000,14.50000,11.00000]

Constraints:

The number of nodes in the tree is in the range [1, 10⁴].
-2³¹ <= Node.val <= 2³¹ - 1

Average of Levels in Binary Tree

Easy

Amazon

3 views

Topics:

Trees

Given the root of a binary tree, return the average value of the nodes on each level in the form of an array. Answers within 10^-5 of the actual answer will be accepted.

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: [3.00000,14.50000,11.00000]
Explanation: The average value of nodes on level 0 is 3, on level 1 is 14.5, and on level 2 is 11.
Hence return [3, 14.5, 11].

Example 2:

Input: root = [3,9,20,15,7]
Output: [3.00000,14.50000,11.00000]

Constraints:

The number of nodes in the tree is in the range [1, 10⁴].
-2³¹ <= Node.val <= 2³¹ - 1

Solution

Clarifying Questions

When you get asked this question in a real-life environment, it will often be ambiguous (especially at FAANG). Make sure to ask these questions in that case:

What is the range of values for the nodes in the binary tree, and can they be negative or floating-point numbers?
What should I return if the input `root` is null or represents an empty tree? Should I return an empty list, or is there a specific value expected?
Is the binary tree guaranteed to be a complete binary tree, or can it be unbalanced?
Are there any memory constraints I should consider, given that I might need to store the nodes at each level during the traversal?
For floating-point averages, what level of precision is required in the output list? Are there specific rounding rules to adhere to?

Brute Force Solution

Approach

The brute force approach to finding the average value at each level of a binary tree involves visiting every single level in the tree. We calculate the sum of all values present at each level. After the sum is calculated, we divide it by the count of nodes at that level to find the average.

Here's how the algorithm would work step-by-step:

Start at the very top of the tree, which is level zero.
Go through the tree level by level. At each level, add up the values of all the 'nodes' you find there.
Count how many 'nodes' there are at each level.
For each level, divide the sum of the values by the number of 'nodes' you counted. This gives you the average value for that level.
Do this for every level of the tree, from top to bottom.
Record the average value for each level.

Code Implementation

def average_of_levels(root):
    if not root:
        return []

    level_averages = []
    nodes_queue = [root]

    while nodes_queue:
        level_length = len(nodes_queue)
        level_sum = 0

        # Iterate over all nodes in current level
        for _ in range(level_length):
            node = nodes_queue.pop(0)
            level_sum += node.val

            # Add children to queue for next level
            if node.left:
                nodes_queue.append(node.left)

            if node.right:
                nodes_queue.append(node.right)

        # Calculate average and record it for level
        average = float(level_sum) / level_length
        level_averages.append(average)

    return level_averages

Big(O) Analysis

Time Complexity

O(n) – The algorithm visits each node in the binary tree exactly once to calculate the sum of node values at each level and the number of nodes at each level. The input size, n, represents the number of nodes in the binary tree. Computing the average for each level then takes O(1) time per level. Therefore, the time complexity is directly proportional to the number of nodes in the tree, resulting in a linear time complexity of O(n).

Space Complexity

O(W) – The brute force approach described uses a level-by-level traversal, implying the use of a queue (or similar data structure) to store the nodes at each level. In the worst-case scenario, the binary tree could be a complete binary tree, where the widest level (W) contains approximately half of all nodes. Therefore, the space complexity is determined by the maximum width of the tree because the queue will hold all nodes at the widest level simultaneously. This means the auxiliary space needed grows linearly with the maximum width (W) of the tree. Hence, the space complexity is O(W).

Optimal Solution

Approach

To find the average value at each level of a binary tree efficiently, we'll visit each level one by one. We'll keep a running sum of the node values and count how many nodes are at that level so we can find the average.

Here's how the algorithm would work step-by-step:

Begin with the very top level of the tree, which only includes the single root node.
Create a 'to do' list that initially contains only the root node.
While there are levels left 'to do' (meaning our 'to do' list isn't empty):
Calculate the sum of all the node values on the current level.
Count how many nodes are on this level.
Compute the average for the level by dividing the sum by the count.
Prepare the 'to do' list for the next level by gathering all the children of the nodes from the current level.
Record the average and repeat the process for the next level.

Code Implementation

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

def averageOfLevels(root):
    levels_averages = []
    nodes_to_do = [root]

    # Process each level of the tree
    while nodes_to_do:

        level_sum = 0
        level_size = 0
        next_level_nodes = []

        # Iterate through the nodes at the current level
        for node in nodes_to_do:
            level_sum += node.val
            level_size += 1

            # Add children to the queue for the next level
            if node.left:
                next_level_nodes.append(node.left)
            if node.right:
                next_level_nodes.append(node.right)

        # Store average value for the current level
        average = float(level_sum) / level_size
        levels_averages.append(average)

        # Update the queue for the next level traversal.
        nodes_to_do = next_level_nodes

    return levels_averages

Big(O) Analysis

Time Complexity

O(n) – The algorithm visits each node in the binary tree exactly once. The outer while loop iterates through each level of the tree, and within each level, the inner operations (summing node values, counting nodes, and collecting children) are performed once per node at that level. Since each node is enqueued and dequeued from the 'to do' list only once, the total number of operations is proportional to the number of nodes in the tree, denoted as n. Therefore, the time complexity is O(n).

Space Complexity

O(W) – The primary auxiliary space is used by the 'to do' list, which holds the nodes for the current level. In the worst-case scenario, this list can contain all nodes at the widest level of the binary tree. If W is the maximum width of the tree, the 'to do' list will hold W nodes. Therefore, the space complexity is proportional to the maximum width W of the tree, and represented as O(W).

Edge Cases

Case	How to Handle
Null or empty tree (root is null)	Return an empty list immediately since there are no levels to average.
Tree with only one node (root only)	Return a list containing only the root's value as the average of level 0.
Tree with all nodes having the same value.	The average at each level will be the same as the node value, handled correctly by level order traversal.
Tree with very large or very small node values, leading to potential integer overflow during sum calculation at a level.	Use long or double to store the sum of node values at each level to prevent overflow.
Highly skewed tree (e.g., all nodes on one branch)	The level order traversal still works correctly, averaging only the node at each level.
Tree with a very large number of levels, potentially exceeding stack space if using recursion.	Use iterative level order traversal (BFS) to avoid stack overflow errors.
Floating-point precision issues when calculating the average.	Use double to store the average and be aware of potential minor inaccuracies.
Negative node values.	The average calculation will handle negative values correctly as part of the sum.

Solution

Clarifying Questions

When you get asked this question in a real-life environment, it will often be ambiguous (especially at FAANG). Make sure to ask these questions in that case:

What is the range of values for the nodes in the binary tree, and can they be negative or floating-point numbers?
What should I return if the input `root` is null or represents an empty tree? Should I return an empty list, or is there a specific value expected?
Is the binary tree guaranteed to be a complete binary tree, or can it be unbalanced?
Are there any memory constraints I should consider, given that I might need to store the nodes at each level during the traversal?
For floating-point averages, what level of precision is required in the output list? Are there specific rounding rules to adhere to?

Brute Force Solution

Approach

Here's how the algorithm would work step-by-step:

Start at the very top of the tree, which is level zero.
Go through the tree level by level. At each level, add up the values of all the 'nodes' you find there.
Count how many 'nodes' there are at each level.
For each level, divide the sum of the values by the number of 'nodes' you counted. This gives you the average value for that level.
Do this for every level of the tree, from top to bottom.
Record the average value for each level.

Code Implementation

def average_of_levels(root):
    if not root:
        return []

    level_averages = []
    nodes_queue = [root]

    while nodes_queue:
        level_length = len(nodes_queue)
        level_sum = 0

        # Iterate over all nodes in current level
        for _ in range(level_length):
            node = nodes_queue.pop(0)
            level_sum += node.val

            # Add children to queue for next level
            if node.left:
                nodes_queue.append(node.left)

            if node.right:
                nodes_queue.append(node.right)

        # Calculate average and record it for level
        average = float(level_sum) / level_length
        level_averages.append(average)

    return level_averages

Big(O) Analysis

Time Complexity

Space Complexity

Optimal Solution

Approach

Here's how the algorithm would work step-by-step:

Begin with the very top level of the tree, which only includes the single root node.
Create a 'to do' list that initially contains only the root node.
While there are levels left 'to do' (meaning our 'to do' list isn't empty):
Calculate the sum of all the node values on the current level.
Count how many nodes are on this level.
Compute the average for the level by dividing the sum by the count.
Prepare the 'to do' list for the next level by gathering all the children of the nodes from the current level.
Record the average and repeat the process for the next level.

Code Implementation

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

def averageOfLevels(root):
    levels_averages = []
    nodes_to_do = [root]

    # Process each level of the tree
    while nodes_to_do:

        level_sum = 0
        level_size = 0
        next_level_nodes = []

        # Iterate through the nodes at the current level
        for node in nodes_to_do:
            level_sum += node.val
            level_size += 1

            # Add children to the queue for the next level
            if node.left:
                next_level_nodes.append(node.left)
            if node.right:
                next_level_nodes.append(node.right)

        # Store average value for the current level
        average = float(level_sum) / level_size
        levels_averages.append(average)

        # Update the queue for the next level traversal.
        nodes_to_do = next_level_nodes

    return levels_averages

Big(O) Analysis

Time Complexity

Space Complexity

Edge Cases

Case	How to Handle
Null or empty tree (root is null)	Return an empty list immediately since there are no levels to average.
Tree with only one node (root only)	Return a list containing only the root's value as the average of level 0.
Tree with all nodes having the same value.	The average at each level will be the same as the node value, handled correctly by level order traversal.
Tree with very large or very small node values, leading to potential integer overflow during sum calculation at a level.	Use long or double to store the sum of node values at each level to prevent overflow.
Highly skewed tree (e.g., all nodes on one branch)	The level order traversal still works correctly, averaging only the node at each level.
Tree with a very large number of levels, potentially exceeding stack space if using recursion.	Use iterative level order traversal (BFS) to avoid stack overflow errors.
Floating-point precision issues when calculating the average.	Use double to store the average and be aware of potential minor inaccuracies.
Negative node values.	The average calculation will handle negative values correctly as part of the sum.