Show all combinations of numbers from 1 to 9 that add up to 100 by adding, subtracting and concatenating

So, this week I saw a blog post somewhere where the poster claimed that if you can't solve the mentioned 5 problems each in under and hour, you ain't no programmer, dev or software engineer. So, I went in and read the problems. 1-4 were easy but number 5 got me a worrying a little. And expected, I failed to solve that in under 1 hour yesterday. But today morning I solved that in less than 30 minutes. But whatever, I can't call myself a programmer anymore :(
Here's my approach though, using recursion:
Each i (from 1 through 9) has 2 different ways of connecting to the sequence.
1. Add itself to the closest sum (1 + 23 + 4 + .......)
2. Or concatenate itself (1 + 234 + ......)
So if we either add an i to the sequence that has reached it or we concatenate. Examples: .......8 + 9
.......89
Now the part before 8 has the same behavior with 8
.......7 + 8
.......78
This goes all the way back to 1 and 2
1 + 2
12
So, the program works something like this

Each branch ultimately ends on the leftmost call.
Code:

#include <cstdio>
#include <iostream>
#include <sstream>

using namespace std;

string i2s(int i) {
    stringstream op;
    op << i;
    return op.str();
}

int solveS(int i, int sum, string p) {

    if (i >= 10) {
        if (sum == 100) {
            cout << p + " = " << sum << endl;
        }
        return 0;
    }

    int iSum = 0;
    for (int j = i ; j<=9 ; j++) {
        iSum = iSum * 10 + j;
        if (i == 1) {
            solveS(j+1, iSum, i2s(iSum));
        } else {
            solveS(j+1, sum+iSum, p + " + " + i2s(iSum));
            solveS(j+1, sum-iSum, p + " - " + i2s(iSum));
        }
    }

    return 0;
}

int main() {

    //freopen("output.txt", "w+", stdout);

    solveS(1, 0, "");

    return 0;
}