Created kmp_algorithm.cpp

C++/Strings/kmp_algorithm.cpp

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+// C++ program for implementation of KMP pattern searching algorithm
+#include <bits/stdc++.h>
+
+void computeLPSArray(char* pat, int M, int* lps);
+
+// Prints occurrences of txt[] in pat[]
+void KMPSearch(char* pat, char* txt)
+{
+	int M = strlen(pat);
+	int N = strlen(txt);
+
+	// create lps[] that will hold the longest prefix suffix
+	// values for pattern
+	int lps[M];
+
+	// Preprocess the pattern (calculate lps[] array)
+	computeLPSArray(pat, M, lps);
+
+	int i = 0; // index for txt[]
+	int j = 0; // index for pat[]
+	while ((N - i) >= (M - j)) {
+		if (pat[j] == txt[i]) {
+			j++;
+			i++;
+		}
+
+		if (j == M) {
+			printf("Found pattern at index %d ", 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;
+		}
+	}
+}
+
+// Fills lps[] for given pattern pat[0..M-1]
+void computeLPSArray(char* pat, int M, int* lps)
+{
+	// length of the previous longest prefix suffix
+	int len = 0;
+
+	lps[0] = 0; // lps[0] is always 0
+
+	// the loop calculates lps[i] for i = 1 to M-1
+	int i = 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] = 0;
+				i++;
+			}
+		}
+	}
+}
+
+// Driver program to test above function
+int main()
+{
+	char txt[] = "CDCDABABDABACCDCDDABABCABABCDCD";
+	char pat[] = "ABABCABAB";
+	KMPSearch(pat, txt);
+	return 0;
+}