fractionToRecurringDecimal.java

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.

Example 1:

Input: numerator = 1, denominator = 2
Output: "0.5"
Example 2:

Input: numerator = 2, denominator = 1
Output: "2"
Example 3:

Input: numerator = 2, denominator = 3
Output: "0.(6)"




The important thing is to consider all edge cases while thinking this problem through, including: negative integer, possible overflow, etc.

Use HashMap to store a remainder and its associated index while doing the division so that whenever a same remainder comes up, we know there is a repeating fractional part.

Please comment if you see something wrong or can be improved. Cheers!




The time com should be O (denominator), cause the mod must be less than denominator. so you can run from 1 to mod times
//TC: O(denomination)
//SC: O(length(divisor aka denominator) for the hashmap
public class Solution {
    public String fractionToDecimal(int numerator, int denominator) {
        if (numerator == 0) {
            return "0";
        }
        StringBuilder res = new StringBuilder();
        // "+" or "-"
        res.append(((numerator > 0) ^ (denominator > 0)) ? "-" : "");  // make this logic clener

        //to handle overflow can make these be longs 
        long num = Math.abs((long)numerator);
        long den = Math.abs((long)denominator);
        
        // integral part
        res.append(num / den);
        num %= den;
        if (num == 0) {
            return res.toString();
        }
        
        // fractional part
        res.append(".");
        HashMap<Long, Integer> map = new HashMap<Long, Integer>();
        map.put(num, res.length());
        while (num != 0) {
            num *= 10;
            res.append(num / den);
            num %= den;
            if (map.containsKey(num)) {
                int index = map.get(num);
                res.insert(index, "(");
                res.append(")");
                break;
            }
            else {
                map.put(num, res.length());
            }
        }
        return res.toString();
    }
}