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

Ry's Git Tutorial BOOK Review

Ry's Git Tutorial BOOK Review There are many good books about git but  Ry's Git Tutorial stands out as an amazing git tutorial book  I will describe below why: http://amzn.to/2pwEuFJ I want to stress out although I'm going to list below what this books talks about that is not the reason you should read this book.   The reason you should read this book is that it's crystal clear about git! it's just an amazing book, it's fun to read, and I truly get to understand git. It very clear, Ry's manages to bring us closer to git by making things really clear. Ry's makes an outstanding effort to bring us from knowing nothing about git to being an expert. The book starts with the real basics like Initializing the repository on an example website, staging files, exploring the repository. Then he moves on to  undoing changes  and he scans various ways you can undo your changes: viewing old revisions, tagging release, undo commit stages etc. Branches