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.

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
struct TreeNode
{
	int val;
	TreeNode *left;
	TreeNode *right;
	TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};
class Solution {
public:
    int sumNumbers(TreeNode *root) {
		if(root == NULL) return 0;
		return dfs(root,"");
    }
private:
	int dfs(TreeNode *root,std::string s)
	{
		if(root == NULL) return 0;
		s += root->val + '0';
		if(root->left == NULL && root->right == NULL) return std::atoi(s.c_str());
		return dfs(root->left,s) + dfs(root->right,s);
	}
};

leetcode - Sum Root to Leaf Numbers