Bit string

input/code.cpp: In function 'int main()':
input/code.cpp:21:29: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i = 0, x = 0; i < S.size(); i++){
                           ~~^~~~~~~~~~

#include <bits/stdc++.h>
#define va first
#define vb second
#define lb lower_bound
#define pb push_back
#define all(v) v.begin(),v.end()
#define sz(v) ((int)(v).size())
#define fio ios_base::sync_with_stdio(0);cin.tie(0);
using namespace std;
using ll = long long;
using lf = long double;
using pii = pair<int,int>;
using pll = pair<ll,ll>;

string S;
int o;

int main(){
    cin >> S;
    ll ans = 0;
    for(int i = 0, x = 0; i < S.size(); i++){
        x ^= (S[i] == '1');
        ans += (x ? o : i+1-o);
        if(x) ++o;
    }
    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