Add C# solutions

Author: hellomrsun

Diff for: October-LeetCoding-Challenge/01-Number-of-Recent-Calls/Number-of-Recent-Calls.cs

@@ -0,0 +1,24 @@
+public class RecentCounter {
+
+    private Queue<int> queue;
+    private int count;
+    public RecentCounter() {
+        count = 0;
+        queue = new Queue<int>();
+    }
+    
+    public int Ping(int t) {
+        queue.Enqueue(t);
+        while(t - queue.FirstOrDefault() > 3000)
+        {
+            queue.Dequeue();
+        }
+        return queue.Count();
+    }
+}
+
+/**
+ * Your RecentCounter object will be instantiated and called as such:
+ * RecentCounter obj = new RecentCounter();
+ * int param_1 = obj.Ping(t);
+ */

Diff for: README.md

@@ -2,6 +2,15 @@
 
 https://leetcode.com
 
+
+## October LeetCoding Challenge
+Click [here](https://leetcode.com/explore/challenge/card/october-leetcoding-challenge/) for problem descriptions.
+
+Solutions in various programming languages are provided. Enjoy it.
+
+1. [Number of Recent Calls](https://github.com/AlgoStudyGroup/Leetcode/tree/master/October-LeetCoding-Challenge/01-Number-of-Recent-Calls) : Queue
+
+
 ## September LeetCoding Challenge
 Click [here](https://leetcode.com/explore/challenge/card/september-leetcoding-challenge) for problem descriptions.
 
@@ -28,6 +37,7 @@ Solutions in various programming languages are provided. Enjoy it.
 19. [Sequential Digits](https://github.com/AlgoStudyGroup/Leetcode/tree/master/September-LeetCoding-Challenge/19-Sequential-Digits)
 23. [Gas Station](https://github.com/AlgoStudyGroup/Leetcode/tree/master/September-LeetCoding-Challenge/23-Gas-Station)
 28. [Subarray Product Less Than K](https://github.com/AlgoStudyGroup/Leetcode/tree/master/September-LeetCoding-Challenge/28-Subarray-Product-Less-Than-K)
+30. [First Missing Positive](https://github.com/AlgoStudyGroup/Leetcode/tree/master/September-LeetCoding-Challenge/30-First-Missing-Positive)
 
 ## August LeetCoding Challenge
 Click [here](https://leetcode.com/explore/challenge/card/august-leetcoding-challenge/) for problem descriptions.

Diff for: September-LeetCoding-Challenge/30-First-Missing-Positive/First-Missing-Positive.cs

@@ -0,0 +1,43 @@
+public class Solution {
+    public int FirstMissingPositive(int[] nums) {
+         int n = nums.Length;
+
+            // 1. mark numbers (num < 0) and (num > n) with a special marker number (n+1) 
+            // (we can ignore those because if all number are > n then we'll simply return 1)
+            for (int i = 0; i < n; i++)
+            {
+                if (nums[i] <= 0 || nums[i] > n)
+                {
+                    nums[i] = n + 1;
+                }
+            }
+            // note: all number in the array are now positive, and on the range 1..n+1
+
+            // 2. mark each cell appearing in the array, by converting the index for that number to negative
+            for (int i = 0; i < n; i++)
+            {
+                int num = Math.Abs(nums[i]);
+                if (num > n)
+                {
+                    continue;
+                }
+                num--; // -1 for zero index based array (so the number 1 will be at pos 0)
+                if (nums[num] > 0)
+                { // prevents double negative operations
+                    nums[num] = -1 * nums[num];
+                }
+            }
+
+            // 3. find the first cell which isn't negative (doesn't appear in the array)
+            for (int i = 0; i < n; i++)
+            {
+                if (nums[i] >= 0)
+                {
+                    return i + 1;
+                }
+            }
+
+            // 4. no positive numbers were found, which means the array contains all numbers 1..n
+            return n + 1;
+    }
+}