166. Fraction to Recurring Decimal

题目：
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)".

解答：
看的discussion, 代码如下：

public String fractionToDecimal(int numerator, int denominator) {
    if (numerator == 0) return "0";
    
    StringBuilder sb = new StringBuilder();
    //判断正负性
    sb.append((numerator > 0) ^ (denominator > 0) ? "-" : "");
    
    long num = Math.abs((long)numerator);
    long deno = Math.abs((long)denominator);
    //加入整数部分
    sb.append(num / deno);
    num %= deno;
    if (num == 0) {
        return sb.toString();
    }
    
    //加入小数部分
    sb.append(".");
    //记录下已经出现过的numerator,当有重复的时候，即从前一个index开始到当前用“（）”包含进去
    Map<Long, Integer> map = new HashMap<Long, Integer>();
    map.put(num, sb.length());
    while (num != 0) {
        num *= 10;
        sb.append(num / deno);
        num %= deno;
        if (map.containsKey(num)) {
            int index = map.get(num);
            sb.insert(index, "(");
            sb.append(")");
            break;
        } else {
            map.put(num, sb.length());
        }
    }
    return sb.toString();
}