a166

README.md

@@ -66,6 +66,8 @@ leetcode 题解
 
 [144. Binary Tree Preorder Traversal](Solution/101-150/144.md)
 
+[166. Fraction to Recurring Decimal](Solution/151-200/166.md)
+
 [168. Excel Sheet Column Title](Solution/151-200/168.md)
 
 [169. Majority Element](Solution/151-200/169.md)

Solution/151-200/166.md

@@ -0,0 +1,46 @@
+#166. Fraction to Recurring Decimal
+[题目链接](https://leetcode.com/problems/fraction-to-recurring-decimal/)
+```java
+public class Solution {
+    public String fractionToDecimal(int numerator, int denominator) {
+        if (numerator == 0)
+            return "0";
+        StringBuilder sb = new StringBuilder();
+        if ((numerator > 0) ^ (denominator > 0)) {
+            sb.append('-');
+        }
+        // 考虑边界值，因为int型负数比正数多一，所以转换为long
+        long num = Math.abs((long)numerator);
+        long den = Math.abs((long)denominator);
+        if (num < den) {
+            sb.append(0);
+        } else {
+            sb.append(num/den);
+        }
+        long rest = num % den;
+        if (rest == 0) {
+            return sb.toString();
+        }
+        sb.append('.');
+        Map<Long, Integer> numMap = new HashMap<>();
+        int repeatFlag = 0;
+        StringBuilder smallSb = new StringBuilder();
+        numMap.put(rest, repeatFlag++);
+        // 求小数部分，把重复余数的位置记录下来，当遇到重复余数是，说明有无限循环部分
+        while (rest != 0) {
+            rest *= 10;
+            smallSb.append(rest / den);
+            rest = rest % den;
+            if (numMap.containsKey(rest)) {
+                int repeatStart = numMap.get(rest);
+                String snum = smallSb.toString();
+                sb.append(snum.substring(0, repeatStart));
+                sb.append('(').append(snum.substring(repeatStart)).append(')');
+                return sb.toString();
+            }
+            numMap.put(rest, repeatFlag++);
+        }
+        return sb.append(smallSb).toString();
+    }
+}
+```