[Function add]

1. Add leetcode solutions.

Author: Seanforfun

README.md

@@ -297,6 +297,12 @@
 
 [165. Compare Version Numbers](https://github.com/Seanforfun/Algorithm-and-Leetcode/blob/master/leetcode/165.%20Compare%20Version%20Numbers.md)
 
+[166. Fraction to Recurring Decimal](https://github.com/Seanforfun/Algorithm-and-Leetcode/blob/master/leetcode/166.%20Fraction%20to%20Recurring%20Decimal.md)
+
+[167. Two Sum II - Input array is sorted](https://github.com/Seanforfun/Algorithm-and-Leetcode/blob/master/leetcode/167.%20Two%20Sum%20II%20-%20Input%20array%20is%20sorted.md)
+
+[168. Excel Sheet Column Title](https://github.com/Seanforfun/Algorithm-and-Leetcode/blob/master/leetcode/168.%20Excel%20Sheet%20Column%20Title.md)
+
 ## Algorithm(4th_Edition)
 Reading notes of book Algorithm(4th Algorithm),ISBN: 9787115293800.
 All java realization codes are placed in different packages.

leetcode/166. Fraction to Recurring Decimal.md

@@ -60,3 +60,44 @@ class Solution {
     }
 }
 ```
+
+### 二刷
+1. 注意int的上下限问题。
+2. 如何退出程序，返回结果：
+    * 余数为0.
+    * 余数不为零，但是余数已经出现过了。这说明会出现循环小数。
+```Java
+class Solution {
+    public String fractionToDecimal(int numerator, int denominator) {
+        if(denominator == 0) return "NaN";
+        Map<Long, Integer> map = new HashMap<>();
+        StringBuilder sb = new StringBuilder();
+        if(numerator > 0 && denominator < 0 || numerator < 0 && denominator > 0) sb.append('-');
+        long aa = Math.abs((long)numerator);
+        long bb = Math.abs((long)denominator);
+        long digit = aa / bb;
+        sb.append(digit);
+        long remain = aa % bb;
+        if(remain == 0) return sb.toString();
+        sb.append(".");
+        int count = 1;
+        while(remain != 0 && !map.containsKey(remain *10)){
+            remain *= 10;
+            map.put(remain, count++);
+            sb.append(Math.abs(remain / bb));
+            remain %= bb;
+        }
+        if(remain != 0){
+            sb.append(")");
+            int val = map.get(remain * 10);
+            String result = sb.toString();
+            int index = result.indexOf('.');
+            sb.insert(val + index, '(');
+        }
+        return sb.toString();
+    }
+}
+```
+
+
+

leetcode/167. Two Sum II - Input array is sorted.md

@@ -64,4 +64,23 @@ class Solution {
         return low >= high ? null : result;
     }
 }
+```
+
+### 二刷
+1. 双指针，一个指向数组头，一个指向数组尾。
+2. 如果sum小了，low指针右移，不然high指针左移。
+```Java
+class Solution {
+    public int[] twoSum(int[] numbers, int target) {
+        int len = numbers.length;
+        int low = 0, high = len - 1;
+        int sum = numbers[low] + numbers[high];
+        while(sum != target){
+            if(sum < target) low++;
+            else high--;
+            sum = numbers[low] + numbers[high];
+        }
+        return new int[]{low + 1, high + 1};
+    }
+}
 ```

leetcode/168. Excel Sheet Column Title.md

@@ -0,0 +1,66 @@
+## 168. Excel Sheet Column Title
+
+### Question
+Given a positive integer, return its corresponding column title as appear in an Excel sheet.
+
+```
+For example:
+
+    1 -> A
+    2 -> B
+    3 -> C
+    ...
+    26 -> Z
+    27 -> AA
+    28 -> AB 
+    ...
+
+Example 1:
+
+Input: 1
+Output: "A"
+
+Example 2:
+
+Input: 28
+Output: "AB"
+
+Example 3:
+
+Input: 701
+Output: "ZY"
+```
+
+### Thinking:
+```Java
+class Solution {
+    public String convertToTitle(int n) {
+        if(n == 0) return "";
+        StringBuilder sb = new StringBuilder();
+        String map = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
+        while(n != 0){
+            sb.append(map.charAt((n - 1) % 26));
+            n --;
+            n /= 26;
+        }
+        return sb.reverse().toString();
+    }
+}
+```
+
+### 二刷
+1. 唯一恶心的就是都是从1开始计数，所以每次计算之前需要-1.
+```Java
+class Solution {
+    public String convertToTitle(int n) {
+        StringBuilder sb = new StringBuilder();
+        while(n != 0){
+            n--;
+            int remain = n % 26;
+            sb.insert(0, (char)(remain + 'A'));
+            n /= 26;
+        }
+        return sb.toString();
+    }
+}
+```