Bit string

#pragma GCC target("tune=native")

#include <bits/stdc++.h>

using namespace std;
using i32 = int32_t;
using i64 = int64_t;

int main() {
    cin.tie(nullptr)->sync_with_stdio(false);

    string a;
    cin >> a;

    i32 n = a.size();
    vector<i32> odd(n + 1), even(n + 1);

    i64 ans = 0;
    for (i32 i = 0; i < n; ++i) {
        if (a[i] == '0') {
            odd[i + 1] = odd[i];
            even[i + 1] = even[i] + 1;
        } else {
            odd[i + 1] = even[i] + 1;
            even[i + 1] = odd[i];
        }

        ans += even[i + 1];
    }

    cout << ans << endl;

#ifdef DEBUG
    main();
#endif
}

Test 1

Group: 1

Verdict: ACCEPTED

Test 2

Group: 1

Verdict: ACCEPTED

Test 3

Group: 1

Verdict: ACCEPTED

Test 4

Group: 1

Verdict: ACCEPTED

Test 5

Group: 1

Verdict: ACCEPTED

Test 6

Group: 1

Verdict: ACCEPTED

Test 7

Group: 1

Verdict: ACCEPTED

Test 8

Group: 2

Verdict: ACCEPTED

Test 9

Group: 2

Verdict: ACCEPTED

Test 10

Group: 2

Verdict: ACCEPTED

Test 11

Group: 2

Verdict: ACCEPTED

Test 12

Group: 2

Verdict: ACCEPTED

Test 13

Group: 2

Verdict: ACCEPTED

Test 14

Group: 2

Verdict: ACCEPTED

Test 15

Group: 3

Verdict: ACCEPTED

Test 16

Group: 3

Verdict: ACCEPTED

Test 17

Group: 3

Verdict: ACCEPTED

Test 18

Group: 3

Verdict: ACCEPTED

Test 19

Group: 3

Verdict: ACCEPTED

Test 20

Group: 3

Verdict: ACCEPTED

Test 21

Group: 3

Verdict: ACCEPTED