Add c# solution

Author: lijiayi0921

June-LeetCoding-Challenge/09-Is-Subsequence/Is-Subsequence.cs

@@ -0,0 +1,41 @@
+public class Solution {
+    
+    // A faster method
+    public bool IsSubsequence(string s, string t) {
+        if (s == "")
+            return true;
+        
+        if (s.Length > t.Length)
+            return false;
+        
+        int inds = 0;
+        foreach (char c in t) {
+            if (s[inds] == c) {
+                inds++;
+                if (inds == s.Length)
+                    return true;
+            }
+        }
+        return false;
+    }
+    
+    // A more general method
+    public bool IsSubsequenceMine(string s, string t) {
+        
+        if (s.Length > t.Length)
+            return false;
+        
+        // the following comparison makes the method faster
+        if (s.Length == t.Length)
+            return s == t;
+        
+        int inds = 0, indt = 0;
+        while (inds < s.Length && indt < t.Length) {
+            if (s[inds] == t[indt]) {
+                inds++;
+            }
+            indt++;
+        }
+        return inds > s.Length-1;
+    }
+}