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)".

public class Solution {
    public String fractionToDecimal(int numerator, int denominator) {
        if(numerator == 0 || denominator == 0) return "0";
        String sign = "";
        if(numerator > 0 && denominator < 0 || numerator < 0 && denominator > 0) sign = "-";
        long num = Math.abs((long) numerator);
        long den = Math.abs((long) denominator);
        long major = num / den;
        long rem = num % den;
        if(rem == 0) return sign + major;
        
        StringBuilder sb = new StringBuilder(sign + major + ".");
        HashMap<Long, Integer> map = new HashMap<>();  // long ： rem  integer 在sb中的pos
        while(rem != 0){
            if(map.containsKey(rem)){    //相同的余数， 循环开始
                int index = map.get(rem);
                sb.insert(index -1 , "(").append(")");  //因为sb中已经包含的循环部分；所以直接插入（）
                break;
            }
            else{
               sb.append(rem * 10 / den);
               map.put((rem) % den , sb.length());
            }
            rem =rem * 10 % den;
        }
        return sb.toString();
    }
}