C++ Programming – number of contiguous subsequences

// C++ program to count substrings with recursive sum equal to 9
#include <iostream>
#include <cstring>
using namespace std;
 
int count9s(char number[])
{
    int count = 0; // To store result
    int n = strlen(number);
 
    // Consider every character as beginning of substring
    for (int i = 0; i < n; i++)
    {
        int sum = number[i] - '0';  //sum of digits in current substring
 
        if (number[i] == '9') count++;
 
        // One by one choose every character as an ending character
        for (int j = i+1; j < n; j++)
        {
            // Add current digit to sum, if sum becomes multiple of 5
            // then increment count. Let us do modular arithmetic to
            // avoid overflow for big strings
            sum = (sum + number[j] - '0')%9;
 
            if (sum == 0)
               count++;
        }
    }
    return count;
}
 
// driver program to test above function
int main()
{
    cout << count9s("4189") << endl;
    cout << count9s("1809");
    return 0;
}