LeetCode:Fraction to Recurring Decimal

题目描述：

Given two integers representing the numerator and denominator of a fraction, return the fraction in string format.

If the fractional part is repeating, enclose the repeating part in parentheses.

For example,

Given numerator = 1, denominator = 2, return "0.5".
Given numerator = 2, denominator = 1, return "2".
Given numerator = 2, denominator = 3, return "0.(6)".

思路分析：先算正数部分，再用for循环算小数部分。在计算的过程中，将所有出现过的小数位放在map中，并在map中记录先他们在结果中的位置。如果当前计算得到的值在map中已经出现了，则表示出现了循环，根据之前记录下的位置在结果中插入括号，即可得最终结果。

代码：

string solution::fractionToDecimal(int64_t numerator,int64_t denominator)
{
string result = "";
if(numerator == 0)
return "0";
if(numerator < 0 ^ denominator < 0)
result += "-";
if(numerator < 0)
numerator = numerator * -1;
if(denominator < 0)
denominator = denominator * -1;
result += to_string(numerator/denominator);
if(numerator % denominator == 0)
return result;
result += ".";
unordered_map<int,int> um;
for(int64_t r = numerator % denominator;r != 0;r = r % denominator)
{
if(um.count(r) > 0)
{
result.insert(um[r],1,'(');
result += ")";
break;
}
um[r] = result.size();
r = r*10;
result +=to_string(r/denominator);
}
return result;
}