Approach

To tackle the problem of calculating the sum of all root-to-leaf numbers in a binary tree, we can break down the process into a structured framework:

1. Understand the Problem: Recognize that each root-to-leaf path represents a number formed by the values of the nodes along that path.

2. Define the Function: Create a recursive function that traverses the tree while keeping track of the current number formed by the path from the root to the current node.

3. Base Case: Identify when a leaf node is reached and add the current number to the total sum.

4. Recursive Case: Continue the traversal for both left and right child nodes.

5. Return the Total Sum: After traversing the tree, return the accumulated sum of all root-to-leaf numbers.

Key Points

• Recursive Traversal: Utilizing recursion simplifies the process of exploring the binary tree.

• Path Tracking: Maintain a variable to track the current number as you traverse down the tree.

• Leaf Node Identification: Ensure to check if a node is a leaf before adding to the sum.

• Efficiency: The algorithm operates in O(N) time complexity, where N is the number of nodes in the tree.

Standard Response

Here is a fully-formed sample answer that illustrates how to write the function:

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

def sumNumbers(root: TreeNode) -> int:
 def dfs(node, current_sum):
 if not node:
 return 0
 
 # Update the current sum by appending the current node's value
 current_sum = current_sum * 10 + node.val
 
 # If it's a leaf node, return the current sum
 if not node.left and not node.right:
 return current_sum
 
 # Recursive case: sum the values from the left and right children
 return dfs(node.left, current_sum) + dfs(node.right, current_sum)
 
 # Start the dfs traversal with an initial sum of 0
 return dfs(root, 0)

• We define a TreeNode class to represent each node in the binary tree.

• The sumNumbers function initializes a depth-first search (DFS) through the tree.

• The inner function dfs calculates the current number as it traverses down each path.

• Once a leaf node is reached, it returns the accumulated sum. Otherwise, it continues the traversal until all paths are evaluated.

• Explanation:

Tips & Variations

Common Mistakes to Avoid:

• Not Handling Edge Cases: Ensure to handle empty trees (i.e., when root is None).

• Incorrect Path Calculation: Be cautious not to reset the path sum incorrectly during recursion.

Alternative Ways to Answer:

• Iterative Approach: Instead of recursion, a stack can be used to perform an iterative traversal of the tree.

Role-Specific Variations:

• Technical Roles: Focus on algorithm efficiency and edge case handling.

• Managerial Roles: Emphasize the importance of clear code and documentation for maintainability.

• Creative Roles: Highlight innovative approaches to visualizing the binary tree and its paths.

Follow-Up Questions

• How would you handle a tree with negative values?

• Can you modify your approach to also return the paths that lead to each root-to-leaf number?

• What would be the time complexity if you were to include additional features, like finding the maximum root-to-leaf number?

By structuring your response in this way, you not only demonstrate your coding skills but also your ability to communicate complex concepts clearly. This is crucial for job seekers in technical interviews and can be adapted based on the role and company culture