/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int sum = 0;
    void track_back(TreeNode* cur , string s)
    {
        if(cur == nullptr ) return;
        s += to_string(cur->val);
        if(cur->left == nullptr && cur->right ==nullptr) sum += stoi(s);
        track_back(cur->left , s);
        track_back(cur->right ,s);
        return;
    }
    int sumNumbers(TreeNode* root) {
        string s;
        track_back(root,s);
        return sum;
    }
};