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

.

public class Solution {
    public int sumNumbers(TreeNode root) {
        return dfs(root,0);
    }
    public int dfs(TreeNode root,int pre){
        if(root == null){
            return 0;
        }
        int sum = root.val + pre*10;
        if(root.left == null && root.right == null){
            return sum;
        }
        return dfs(root.left,sum)+dfs(root.right,sum);
    }
}