Leetcode: 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.

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.

Solution:

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    int sum;
    public int sumNumbers(TreeNode root) {
        sum = 0;
        sumNumbers(root, 0);
        return sum;
    }
    public void sumNumbers(TreeNode root, int partialSum) 
    {
        if(root == null)
            return;
        partialSum = partialSum*10 + root.val;
        if (root.left==null && root.right == null)
            sum += partialSum;
        else
        {
            sumNumbers(root.left, partialSum);
            sumNumbers(root.right, partialSum);
        }
    }
}