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.

Analyse: DFS. Compute the left and right subtree of the root respectively and add them. 

Runtime: 4ms.

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int sumNumbers(TreeNode* root) {
        return dfs(root, 0);
    }
    int dfs(TreeNode* root, int sum){
        if(!root) return 0;
        if(!root->left && !root->right) return sum * 10 + root->val;
        
        return dfs(root->left, sum * 10 + root->val) +
               dfs(root->right, sum * 10 + root->val);
    }
};