Sum Root to Leaf Numbers

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.

Example:

Input: [1,2,3]
    1
   / \
  2   3
Output: 25
Explanation:
The root-to-leaf path 1->2 represents the number 12.
The root-to-leaf path 1->3 represents the number 13.
Therefore, sum = 12 + 13 = 25.

Ans:

public int sumNumbers(TreeNode root) {
        return sumNumbersHelper(root,0);
    }
    
    private int sumNumbersHelper(TreeNode T, int sum) {
        if(T==null){
            return 0;
        }
        sum = sum*10+T.val;
        // leaf
        if(T.left == null && T.right == null) {
            return sum;
        }
        // non leaf
        return sumNumbersHelper(T.left, sum) + sumNumbersHelper(T.right, sum);
    }