129 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.
Solution

/**
 * 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:
    vector<string> helper(TreeNode* root) {
        vector<string> res;
        if ( root == NULL ) return res;
        if ( root->left == NULL and root->right == NULL) { res.push_back(to_string(root->val)); return res;}
        vector<string> left = helper(root->left);
        vector<string> right = helper(root->right);
        int l = left.size(), r = right.size();
        for ( int i = 0; i <= l-1; i++ ) {
            string s = to_string(root->val) + left[i];
            res.push_back(s);
        }
        for ( int i = 0; i <= r-1; i++ ) {
            string s = to_string(root->val) + right[i];
            res.push_back(s);
        }
        return res;

    }
    int sumNumbers(TreeNode* root) {
        vector<string> res = helper(root);
        int sum = 0, n = res.size();
        for ( int i = 0; i<= n-1; i++ ) sum += atoi(res[i].c_str());
        return sum;
    }
};

Notes
DETAILS

• distinguish a leaf node! For example:

    1
   / \
  2   3
   \
    4
  

  The result is 124 + 13 = 137. (12 cannot be counted as a number here.)

• Note the appearance of zero! For example:

    1
   / \
  2   3
   \
    0
  

  If only record an integer in the array, 120 may be mis-represented as 12. So note the trick to record a vector of strings!