Sum Root to Leaf Numbers(Java and Python)

Given a binary tree containing digits from 0-9 only, each root-to-leaf path could represent a number.

An example is the root-to-leaf path 1->2->3 which represents the number 123.

Find the total sum of all root-to-leaf numbers.

For example,

    1
   / \
  2   3

The root-to-leaf path 1->2 represents the number 12.
The root-to-leaf path 1->3 represents the number 13.

Return the sum = 12 + 13 = 25.

When I met this question, I intuitively come out that recursion may be a good way to solve this problem. For a recursive solution, the base case is extremely important and it is always the key point to solve a question.

Depend on my experience, for a tree question, the root==null is always a base case, another base case in this question is decided when the node is a leaf node. Then we know that we can use root.left==null && root.right==null to represent this situation and it also should be a base case here. Once I found the base cases, we can just apply the DFS to track each path from root to leaf and update the sum variable. The reason I use an array to hold the same value is because Java can only passed by value. For example, 

in a=1; void calculate(int a){ a=a+1; } System.out.println(a); // the result is still 1 , not 2 In order to track sum value of each calculation and not return the value of sum, we should use a collection or a wrapper class to hold it  

So I use an array here to hold this sum value. If you anybody has another way to solve this problem, please leave a comment for me.