Bittijono

#include <iostream>

using namespace std;

const int N = 5e5 + 5;

typedef long long ll;

ll o[N]; //amount of ones <= i
ll even[N]; //amount of indexes <= i with o[j] even
ll odd[N]; //amount of indexes <= i with o[j] odd



int main() {
    string s;
    cin >> s;
    int n = s.length();
    
    if(s[0] == '1') {
        o[1] = 1;
        odd[1] = 1;
    } else {
        even[1] = 1;
    }
    for(int i = 2; i <= n; ++i) {
        if(s[i-1] == '0') {
            o[i] = o[i-1];
        } else {
            o[i] = o[i-1] + 1;
        }
        
        if(o[i]%2 == 0) {
            even[i] = even[i-1] + 1;
            odd[i] = odd[i-1];
        } else {
            even[i] = even[i-1];
            odd[i] = odd[i-1] + 1;
        }
    }
    
    ll ans = 0;
    for(int i = 1; i <= n; ++i) {
        if(o[i]%2 == 1) {
            ans += (ll) odd[i] - 1;
        } else {
            ans += (ll) even[i];
        }
    }
    
    cout << ans << "\n";
}

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