本篇题解有误，经阎帝检查发现35行少一个$t++$，一下是正确代码

#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 10;
int a[N];
map<int, int> mp;
int check(int x) {
    if (x == 0) return 1;
    int d = 0; while (x > 0) x /= 10, d++;
    return d;
}

int main() {
    int n, m; scanf("%d%d", &n, &m);
    if (n % m == 0) { printf("%d.0", n / m); return 0; }
    
    printf("%d.", n / m);
    int t = check(n / m) + 1;
    n %= m;

    int len = 0;
    while (!mp[n] && n) {
        mp[n] = len + 1;
        a[++len] = n * 10 / m;
        n = n * 10 % m;
    }
    
    if (n == 0) {
        for (int i = 1; i <= len; i++) {
            printf("%d", a[i]);
            if ((i + t) % 76 == 0) printf("\n");
        }
    } else {
        int p = mp[n];
        for (int i = 1; i <= len; i++) {
            if (i == p) printf("("),t++;
            printf("%d", a[i]);
            if ((i + t) % 76 == 0) printf("\n");
        }
        printf(")\n");
    }
    return 0;
}

#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 10;
int a[N];
unordered_map<int, int> mp;

int check(int x) {
    if (x == 0) return 1;
    int d = 0; while (x > 0) x /= 10, d++;
    return d;
}

int main() {
    int n, m; scanf("%d%d", &n, &m);
    if (n % m == 0) { printf("%d.0", n / m); return 0; }
    
    printf("%d.", n / m);
    int t = check(n / m) + 1;
    n %= m;

    int len = 0;
    while (!mp[n] && n) {
        mp[n] = len + 1;
        a[++len] = n * 10 / m;
        n = n * 10 % m;
    }
    
    if (n == 0) {
        for (int i = 1; i <= len; i++) {
            printf("%d", a[i]);
            if ((i + t) % 76 == 0) printf("\n");
        }
    } else {
        int p = mp[n];
        for (int i = 1; i <= len; i++) {
            if (i == p) printf("(");
            printf("%d", a[i]);
            if ((i + t) % 76 == 0) printf("\n");
        }
        printf(")\n");
    }
    return 0;
}