166. 分数到小数

给定两个整数，分别表示分数的分子 numerator 和分母 denominator，以 字符串形式返回小数 。

如果小数部分为循环小数，则将循环的部分括在括号内。

如果存在多个答案，只需返回 任意一个 。

思路:模拟除法,借助哈希表来判断是否出现循环

代码如下:
 

public String fractionToDecimal(int numerator, int denominator) {
    long numeratorLong = numerator;
    long denominatorLong = denominator;
    if (numeratorLong % denominatorLong == 0){
        return String.valueOf(numeratorLong / denominatorLong);
    }
    StringBuffer sb = new StringBuffer();
    if (numeratorLong < 0 ^ denominatorLong < 0){
        sb.append('-');
    }

    //整数部分
    numeratorLong = Math.abs(numeratorLong);
    denominatorLong = Math.abs(denominatorLong);
    long integerPart = numeratorLong / denominatorLong;
    sb.append(integerPart);
    sb.append('.');

    //小数部分
    StringBuffer fractionPart = new StringBuffer();
    HashMap map = new HashMap<>();
    long remainder = numeratorLong % denominatorLong;
    int index = 0;
    while (remainder != 0 && !map.containsKey(remainder)){
        map.put(remainder, index);
        remainder = remainder * 10;
        fractionPart.append(remainder / denominatorLong);
        remainder = remainder % denominatorLong;
        index++;
    }
    if (remainder != 0){
        Integer insertIndex = map.get(remainder);
        fractionPart.insert(insertIndex, "(");
        fractionPart.append(')');
    }
    sb.append(fractionPart);
    return sb.toString();
}

结果: