Root to leaf paths sum

Given a binary tree of N nodes, where every node value is a number. Find the sum of all the numbers which are formed from root to leaf paths.

• Time : O(n)
• Space : O(d)

void helper(Node* node, long long path, long long& res) {
    if(!node) return;
    path = path * 10 + node->data;
    if(!node->left and !node->right) {
        res += path;
    } else {
        helper(node->left, path, res);
        helper(node->right, path, res);
    }
}
long long treePathsSum(Node *root)
{
    long long res = 0;
    helper(root, 0, res);
    return res;
}