思路：
模拟。
实现：
1 class Solution 2 { 3 public: 4 string fractionToDecimal(int numerator, int denominator) 5 { 6 long long a = numerator, b = denominator; 7 string ans = ""; 8 if (a * b < 0) ans += '-'; 9 a = abs(a); b = abs(b); 10 ans += to_string(a / b); 11 a %= b; 12 if (!a) return ans; 13 ans += '.'; 14 a *= 10; 15 vector<long long> v; 16 map<pair<long long, long long>, int> mp; 17 long long rem = a % b, q = a / b; 18 while (rem && !mp.count(make_pair(rem, q))) 19 { 20 v.push_back(q); 21 mp[make_pair(rem, q)] = v.size() - 1; 22 rem *= 10; 23 q = rem / b; 24 rem %= b; 25 } 26 if (rem == 0) 27 { 28 v.push_back(q); 29 for (auto it: v) ans += to_string(it); 30 } 31 else 32 { 33 int start = mp[make_pair(rem, q)]; 34 for (int i = 0; i < start; i++) ans += to_string(v[i]); 35 ans += '('; 36 for (int i = start; i < v.size(); i++) ans += to_string(v[i]); 37 ans += ')'; 38 } 39 return ans; 40 } 41 };