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)".

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Credits:
Special thanks to @Shangrila for adding this problem and creating all test cases.

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这道题答案不是我自己想的，答案写的很清楚，把每次的余数存起来之后用HashMap查找这个余数之前有没有出现过是亮点

public class Solution {      public String fractionToDecimal(int numerator, int denominator) {          if (numerator == 0) return "0";          if (denominator == 0) return "";                    String ans = "";                    //如果结果为负数          if ((numerator < 0) ^ (denominator < 0)) {              ans += "-";          }                    //下面要把两个数都转为正数，为避免溢出，int转为long          long num = numerator, den = denominator;          num = Math.abs(num);          den = Math.abs(den);                    //结果的整数部分          long res = num / den;          ans += String.valueOf(res);                    //如果能够整除，返回结果          long rem = (num % den) * 10;          if (rem == 0) return ans;                    //结果的小数部分          HashMap<Long, Integer> map = new HashMap<Long, Integer>();          ans += ".";          while (rem != 0) {              //如果前面已经出现过该余数，那么将会开始循环              if (map.containsKey(rem)) {                  int beg = map.get(rem); //循环体开始的位置                  String part1 = ans.substring(0, beg);                  String part2 = ans.substring(beg, ans.length());                  ans = part1 + "(" + part2 + ")";                  return ans;              }                            //继续往下除              map.put(rem, ans.length());              res = rem / den;              ans += String.valueOf(res);              rem = (rem % den) * 10;          }                    return ans;      }  }