分数到小数

问题描述

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。

示例 1:
输入: numerator = 1, denominator = 2
输出: "0.5"

示例 2:
输入: numerator = 2, denominator = 1
输出: "2"

示例 3:
输入: numerator = 2, denominator = 3
输出: "0.(6)"

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/fraction-to-recurring-decimal

思路

在小数部分如果出现两次除数相同的情况即说明是循环小数.
来源：https://leetcode-cn.com/problems/fraction-to-recurring-decimal/solution/li-yong-hashbiao-ding-wei-xun-huan-by-just_coding/

题解

class Solution {
public:
    string fractionToDecimal(int numerator, int denominator) {
        string ret;
        if (denominator == 0) return ret;
        if (numerator == 0) return "0";
        long long num = numerator;
        long long denom = denominator;

        if (num < 0 && denom > 0 || num > 0 && denom < 0) {
            ret += "-";
        }
        num = abs(num);
        denom = abs(denom);

        ret += to_string(num / denom);
        num = num % denom;

        if (num != 0) {
            ret += ".";
        }
        map<long long, int> table;
        int index = 0;
        while (num != 0 && table.find(num) == table.end()) {
            table[num] = index++;
            num *= 10;
            ret += to_string(num / denom);
            num %= denom;
        }
        if (table.find(num) != table.end()) {
            ret += "()";
            int cur = ret.size() - 2;
            while (index-- > table[num]) {
                swap(ret[cur], ret[cur - 1]);
                --cur;
            }
        }
        return ret;
    }
};