Skip to main content

LeetCode 129 Sum Root to Leaf Number

 

LeetCode 129 Sum Root to Leaf Numbers

Given a root of binary tree containing digits from 0 to 9 only. Return the total sum of all root to leaf.

The question is taken from the book “Elements of Programming Interviews in Python” page 125 https://amzn.to/3y84FHQ

We see we have here these routes to path

  1. 495
  2. +491
  3. +40

Which in total is 1026

Now let’s see how to solve it in java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    
    // This is an excellent trick to save us from passing params.
    int total = 0;
    
    public int sumNumbers(TreeNode root) {
        sumNumbers(root, 0);
        return total;
    }
    
    private void sumNumbers(TreeNode root, int singlePathSum) {
        
        // Step 1: Identify *no node* if no node given to us do nothing.
        if (root == null) return;
        
        // Step 2: Identify *leaf* node add the leaf node and return total.
        if (root.left == null && root.right == null) {
            total = total + (singlePathSum * 10 + root.val);
            return;
        }
        
        // Step 3: Mid tree - calc right and left.
        sumNumbers(root.left, root.val + singlePathSum * 10);
        sumNumbers(root.right, root.val + singlePathSum * 10);
        
        return;

    } 
}

As we see we have a few steps

  1. First we use a trick to hold on to total in a member variable, this helps it with the recursion.
  2. Step 1 we identify the extreme case of having no real root we do nothing nothing to add here.
  3. Step 2 We identify a leaf node and add it to total and multiply the so far by 10.
  4. Step 3 We identify we are in middle of tree and do the recursion on right and left, we let those recursion steps update the total.

To see the full question go to page 125 on book below which I think is the best book for learning programming interviews

Comments

Popular posts from this blog

Dev OnCall Patterns

Introduction Being On-Call is not easy. So does writing software. Being On-Call is not just a magic solution, anyone who has been On-Call can tell you that, it's a stressful, you could be woken up at the middle of the night, and be undress stress, there are way's to mitigate that. White having software developers as On-Calls has its benefits, in order to preserve the benefits you should take special measurements in order to mitigate the stress and lack of sleep missing work-life balance that comes along with it. Many software developers can tell you that even if they were not being contacted the thought of being available 24/7 had its toll on them. But on the contrary a software developer who is an On-Call's gains many insights into troubleshooting, responsibility and deeper understanding of the code that he and his peers wrote. Being an On-Call all has become a natural part of software development. Please note I do not call software development software engineering b

SQL Window functions (OVER, PARTITION_BY, ...)

Introduction When you run an SQL Query you select rows, but what if you want to have a summary per multiple rows, for example you want to get the top basketball for each country, in this case we don't only group by country, but we want also to get the top player for each of the country.  This means we want to group by country and then select the first player.  In standard SQL we do this with joining with same table, but we could also use partition by and windowing functions. For each row the window function is computed across the rows that fall into the same partition as the current row.  Window functions are permitted only in the  SELECT  list and the  ORDER BY  clause of the query They are forbidden elsewhere, such as in  GROUP BY ,  HAVING  and  WHERE  clauses. This is because they logically execute after the processing of those clauses Over, Partition By So in order to do a window we need this input: - How do we want to group the data which windows do we want to have? so  def c

Building Secure and Reliable Systems

A recent book was published this year by Google about site reliability and security engineering, I would like to provide you a brief overview of it and incorporate my own analysis and thoughts about this subject while saving you some time from reading, at least part of it. Take a few of your customers and ask them, what are the top 5 features on my product that you like.  The answer that you are likely to get is, I really like how polished the UI is, or the daily report I get by mail is just fantastic, or since I started using your product I was able to save one hour a day my productivity got up and the share /chat button on document that you added recently is doing a great job. Your customers are very unlikely to answer the question of what top 5 features of my product do you like with I really like its security or I really like that we lost no chat messages since I started using it.  No real customer will even think of it, moreover, assuming you did a very good job, they won&#