Add C# solutions

Author: hellomrsun

README.md

@@ -24,6 +24,8 @@ Solutions in various programming languages are provided. Enjoy it.
 15. [Length of Last Word](https://github.com/AlgoStudyGroup/Leetcode/tree/master/September-LeetCoding-Challenge/15-Length-of-Last-Word)
 16. [Maximum XOR of Two Numbers in an Array](https://github.com/AlgoStudyGroup/Leetcode/tree/master/September-LeetCoding-Challenge/16-Maximum-XOR-of-Two-Numbers-in-an-Array)
 17. [Robot Bounded In Circle](https://github.com/AlgoStudyGroup/Leetcode/tree/master/September-LeetCoding-Challenge/17-Robot-Bounded-In-Circle)
+18. [Best Time to Buy and Sell Stock](https://github.com/AlgoStudyGroup/Leetcode/tree/master/September-LeetCoding-Challenge/18-Best-Time-to-Buy-and-Sell-Stock)
+19. [Sequential Digits](https://github.com/AlgoStudyGroup/Leetcode/tree/master/September-LeetCoding-Challenge/19-Sequential-Digits)
 
 ## August LeetCoding Challenge
 Click [here](https://leetcode.com/explore/challenge/card/august-leetcoding-challenge/) for problem descriptions.

September-LeetCoding-Challenge/18-Best-Time-to-Buy-and-Sell-Stock/Best-Time-to-Buy-and-Sell-Stock.cs

@@ -0,0 +1,11 @@
+public class Solution {
+    public int MaxProfit(int[] prices) {
+        if(prices == null || prices.Length == 0) return 0;
+        int min=prices[0],max=0;
+        for (int i=1;i<prices.Length;i++){
+            min=Math.Min(min,prices[i]);
+            max=Math.Max(max,prices[i]-min);
+        }
+        return max;
+    }
+}

September-LeetCoding-Challenge/19-Sequential-Digits/Sequential-Digits.cs

@@ -0,0 +1,20 @@
+public class Solution {
+    public IList<int> SequentialDigits(int low, int high) {
+        int[] allNums = {12,23,34,45,56,67,78,89,
+                         123,234,345,456,567,678,789,
+                         1234,2345,3456,4567,5678,6789,
+                         12345,23456,34567,45678,56789,
+                         123456,234567,345678,456789,
+                         1234567,2345678,3456789,
+                         12345678,23456789,
+                         123456789};
+        var res = new List<int>();
+        int n = allNums.Length;
+        for (int i = 0; i < n; i++) {
+            if (allNums[i] < low) continue;
+            if (allNums[i] > high) break;
+            res.Add(allNums[i]);
+        }
+        return res;
+    }
+}