Add C# code for string matching KMP Algo

String/KMP/KMP.cs

@@ -0,0 +1,88 @@
+// C# program for implementation of KMP pattern 
+// searching algorithm 
+using System; 
+
+class GFG { 
+
+	void KMPSearch(string pat, string txt) 
+	{ 
+		int M = pat.Length; 
+		int N = txt.Length; 
+
+		// create lps[] that will hold the longest 
+		// prefix suffix values for pattern 
+		int[] lps = new int[M]; 
+		int j = 0; // index for pat[] 
+
+		// Preprocess the pattern (calculate lps[] 
+		// array) 
+		computeLPSArray(pat, M, lps); 
+
+		int i = 0; // index for txt[] 
+		while (i < N) { 
+			if (pat[j] == txt[i]) { 
+				j++; 
+				i++; 
+			} 
+			if (j == M) { 
+				Console.Write("Found pattern "
+							+ "at index " + (i - j)); 
+				j = lps[j - 1]; 
+			} 
+
+			// mismatch after j matches 
+			else if (i < N && pat[j] != txt[i]) { 
+				// Do not match lps[0..lps[j-1]] characters, 
+				// they will match anyway 
+				if (j != 0) 
+					j = lps[j - 1]; 
+				else
+					i = i + 1; 
+			} 
+		} 
+	} 
+
+	void computeLPSArray(string pat, int M, int[] lps) 
+	{ 
+		// length of the previous longest prefix suffix 
+		int len = 0; 
+		int i = 1; 
+		lps[0] = 0; // lps[0] is always 0 
+
+		// the loop calculates lps[i] for i = 1 to M-1 
+		while (i < M) { 
+			if (pat[i] == pat[len]) { 
+				len++; 
+				lps[i] = len; 
+				i++; 
+			} 
+			else // (pat[i] != pat[len]) 
+			{ 
+				// This is tricky. Consider the example. 
+				// AAACAAAA and i = 7. The idea is similar 
+				// to search step. 
+				if (len != 0) { 
+					len = lps[len - 1]; 
+
+					// Also, note that we do not increment 
+					// i here 
+				} 
+				else // if (len == 0) 
+				{ 
+					lps[i] = len; 
+					i++; 
+				} 
+			} 
+		} 
+	} 
+
+	// Driver program to test above function 
+	public static void Main() 
+	{ 
+		string txt = "ABABDABACDABABCABAB"; 
+		string pat = "ABABCABAB"; 
+		new GFG().KMPSearch(pat, txt); 
+	} 
+} 
+
+// This code has been contributed by Amit Khandelwal.