Add Power of Four C#

Author: hellomrsun

August-LeetCoding-Challenge/04-Power-of-Four/Power-of-Four.cs

@@ -0,0 +1,20 @@
+public class Solution {
+    //Solution 1
+    public bool IsPowerOfFour(int num) {       
+        if(num > 1)
+        {
+            while(num % 4 == 0){
+                num /= 4;
+            }
+        }
+        return num == 1;
+    }
+
+    //Solution 2
+    public bool IsPowerOfFour(int num) { 
+        //0x55555555 (hex) means 0101 0101 0101 0101 0101 0101 0101 0101 (binary)
+        return (num > 0)
+            && (num & (num-1)) == 0
+            && ((num & 0x55555555) != 0);
+    }
+}

README.md

@@ -10,6 +10,7 @@ Solutions in various programming languages are provided. Enjoy it.
 1. [Detect Capital](https://github.com/AlgoStudyGroup/Leetcode/tree/master/August-LeetCoding-Challenge/01-Detect-Capital)
 2. [Design HashSet](https://github.com/AlgoStudyGroup/Leetcode/tree/master/August-LeetCoding-Challenge/02-Design-HashSet)
 3. [Valid Palindrome](https://github.com/AlgoStudyGroup/Leetcode/tree/master/August-LeetCoding-Challenge/03-Valid-Palindrome)
+4. [Power of Four](https://github.com/AlgoStudyGroup/Leetcode/tree/master/August-LeetCoding-Challenge/04-Power-of-Four)
 
 ## July LeetCoding Challenge
 Click [here](https://leetcode.com/explore/featured/card/july-leetcoding-challenge/) for problem descriptions.