*Sum root to leaf number

题目：

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 

public class Solution {
    public int sumNumbers(TreeNode root) {
        return dfs(root, 0);
    }

    private int dfs(TreeNode root, int prev){
        if(root == null) {
            return 0;
        }

        int sum = root.val + prev * 10;
        if(root.left == null && root.right == null) {
            return sum;
        }

        return dfs(root.left, sum) + dfs(root.right, sum);
    }
}