Good subarrays, almost same code gives wrong output

Debugging Help
#1

Problem Info

Good subarrays: Problem - C - Codeforces

Question

My code is almost the same as the solution by Matthew chen but somehow mine is giving the wrong answer for test 3 (N= 100000, string = all 1s). I have no idea where I made a mistake.

My Work

#include <bits/stdc++.h>
using namespace std;

int main() {
    size_t T; cin >> T;
    for (size_t i=0; i<T; ++i){
        size_t N; cin >> N;
        ssize_t prefix = 0;
        map<ssize_t, ssize_t> prefixes;
        prefixes[0] = 1;
        size_t count = 0;
        for (size_t j=0; j<N; ++j){
            char dig; cin >> dig;
            dig -= '1';
            prefix = prefix + dig;
            count += prefixes[prefix];
            ++prefixes[prefix];
        }
        cout << count << "\n";
    }
}

The solution code is :

// key idea is the following: we subtract one from each element of a[n] to form a new array b[n]
// If a subarray is good in the original array a, then since its sum is equal to its length,
// the subarray is now good if its sum is 0 in b.
#include <bits/stdc++.h>
using namespace std;
int main() {
	const int MN = 1e5+10;
	int n, t, a[MN];
	char x;
	cin >> t;
	for (int i=0; i<t; i++) {
		cin >> n;
		for (int k=0; k<n; k++) {
			cin >> x;
			a[k] = x-'1';
		}
		map<int, int> sums;
		sums[0] = 1;
		long long psum = 0, answer = 0;
		for (int j=0; j<n; j++) {
			psum+=a[j];
			answer += sums[psum];
			sums[psum]++;
		}
		cout << answer << endl;
		memset(a, 0, n+1), sums.clear();
	}
}