166. Fraction to Recurring Decimal

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)".

Hint:

1. No scary math, just apply elementary math knowledge. Still remember how to perform a long division?
2. Try a long division on 4/9, the repeating part is obvious. Now try 4/333. Do you see a pattern?
3. Be wary of edge cases! List out as many test cases as you can think of and test your code thoroughly.

本题之所以AC率如此的低是因为有很多的陷阱：int要转换成long型，并且要先转换再绝对值。思路是这样的，创建一个StringBuilder，然后判断符号，如果为负数，则把“-”输入到动态字符串里面，然后让被除数除以除数，商输入到动态字符串里面，检测余数，如果余数为0，则返回，否则，创建一个hashmap，用来存储余数，进入循环，将余数*10/除数存到动态字符串里面，看其余数，如果余数在hashmap里面出现，则说明是循环的，加入括号；否则，map里面加入余数；代码如下：

public class Solution {
    public String fractionToDecimal(int numerator, int denominator) {
        if(numerator==0) return "0";
        long num = Math.abs((long)numerator);
        long den = Math.abs((long)denominator);
        StringBuilder sb = new StringBuilder();
        sb.append((numerator>0)^(denominator>0)?"-":"");
        sb.append(num/den);
        num = num%den;
        if(num==0) return sb.toString();
        sb.append(".");
        Map<Long,Integer> map = new HashMap<Long,Integer>();
        map.put(num,sb.length());
        while(num!=0){
            num = num*10;
            sb.append(num/den);
            num = num%den;
            if(map.containsKey(num)){
                int index = map.get(num);
                sb.insert(index,"(");
                sb.append(")");
                break;
            }
            map.put(num,sb.length());
        }
        return sb.toString();
    }
}