如何避免精度误差？

#include<bits/stdc++.h>
using namespace std;
// ifstream fin("data.in");
// #define cin fin
// ofstream fout("data.out");
// #define cout fout
#define endl '\n'

const int N = 1e5 + 5;
int a[N], b[N], lst[2];

signed main(){
    ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
    int n; cin >> n;
    for(int i = 1; i <= n; i++) cin >> a[i];
    double x = 0, y = 0, z = 0; 
    for(int k = 30; ~k; k--){
        memset(lst, 0, sizeof lst);
        int c1 = 0, c2 = 0;
        for(int i = 1; i <= n; i++){
            b[i] = a[i] >> k & 1;
            x += (1 << k) * b[i] * (1.0 / (1LL * n * n));
            y += (1 << k) * b[i] * (1.0 / (1LL * n * n));
            z += (1 << k) * b[i] * (1.0 / (1LL * n * n));
            if(b[i]){
                x += (1LL << k) * c1 * 2.0 / (1LL * n * n);
                y += (1LL << k) * (i - 1 - lst[0]) * 2.0 / (1LL * n * n);
                z += (1LL << k) * (i - 1) * 2.0 / (1LL * n * n);
            } else {
                x += (1LL << k) * c2 * 2.0 / (1LL * n * n);
                z += (1LL << k) * lst[1] * 2.0 / (1LL * n * n);
            }
            lst[b[i]] = i, c1++;
            if(b[i]) swap(c1, c2);
        }
    }
    cout << fixed << setprecision(3);
    cout << x << ' ';
    cout << y << ' ';
    cout << z << endl;
    return 0;
}

#include<bits/stdc++.h>
using namespace std;
// ifstream fin("data.in");
// #define cin fin
// ofstream fout("data.out");
// #define cout fout
#define endl '\n'

const int N = 1e5 + 5;
int a[N], b[N], lst[2];

signed main(){
    ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
    int n; cin >> n;
    for(int i = 1; i <= n; i++) cin >> a[i];
    double x = 0, y = 0, z = 0; 
    for(int k = 30; ~k; k--){
        memset(lst, 0, sizeof lst);
        int c1 = 0, c2 = 0;
        for(int i = 1; i <= n; i++){
            b[i] = a[i] >> k & 1;
            double ctb = 1.0 * (1LL << k) / (1LL * n * n);
            if(b[i]){
                x += ctb + 2 * ctb * c1;
                y += ctb + 2 * ctb * (i - 1 - lst[0]);
                z += ctb + 2 * ctb * (i - 1);
            } else {
                x += 2 * ctb * c2;
                z += 2 * ctb * lst[1];
            }
            lst[b[i]] = i, c1++;
            if(b[i]) swap(c1, c2);
        }
    }
    cout << fixed << setprecision(3);
    cout << x << ' ';
    cout << y << ' ';
    cout << z << endl;
    return 0;
}