Solution.java

public class Solution {
    public String fractionToDecimal(int numerator, int denominator) {
        long n=numerator;
        long d=denominator;
        if (d==0) {
            return null;
        }
        if (n==0) {
            return "0";
        }
        if (n%d==0) {
            return String.valueOf(n/d);
        }
        boolean flag=true;
        if (n<0) {
            n=0-n;
            flag=!flag;
        }
        if (d<0) {
            d=0-d;
            flag=!flag;
        }
        Map<Long, Integer> map=new HashMap<Long, Integer>();
        StringBuffer sb=new StringBuffer();
        sb.append(String.valueOf(n/d)).append(".");
        n%=d;
        int i=1;
        map.put(n, i);
        while (true) {
            n*=10;
            sb.append(String.valueOf(n/d));
            n%=d;
            if (n==0) {
                return flag?sb.toString():"-"+sb.toString();
            }
            i++;
            if (map.get(n)==null) {
                map.put(n, i);
            }else {
                String str=sb.toString();
                String circle=str.substring(str.length()-(i-Integer.valueOf(map.get(n))), str.length());
                String answer=str.substring(0,str.length()-(i-Integer.valueOf(map.get(n))))+"("+circle+")";
                return flag?answer:"-"+answer;
            }
        }
    }
}