Add KnuthMorrisPratt.java (KMP)

Author: sriieeu

src/main/java/com/thealgorithms/strings/KnuthMorrisPratt.java

@@ -0,0 +1,74 @@
+package com.thealgorithms.strings;
+
+import java.util.ArrayList;
+import java.util.List;
+
+
+final class KnuthMorrisPratt {
+	private KnuthMorrisPratt() {
+	}
+
+	// Compute the longest proper prefix which is also suffix (LPS) array
+	public static int[] computeLps(final String pattern) {
+		final int n = pattern.length();
+		final int[] lps = new int[n];
+		int len = 0; // length of the previous longest prefix suffix
+		lps[0] = 0;
+		for (int i = 1; i < n; ) {
+			if (pattern.charAt(i) == pattern.charAt(len)) {
+				len++;
+				lps[i] = len;
+				i++;
+			} else {
+				if (len != 0) {
+					len = lps[len - 1];
+				} else {
+					lps[i] = 0;
+					i++;
+				}
+			}
+		}
+		return lps;
+	}
+
+	// Return list of start indices where pattern occurs in text
+	public static List<Integer> search(final String text, final String pattern) {
+		final List<Integer> occurrences = new ArrayList<>();
+		if (pattern == null || pattern.isEmpty() || text == null) {
+			return occurrences;
+		}
+
+		final int[] lps = computeLps(pattern);
+		int i = 0; // index for text
+		int j = 0; // index for pattern
+		final int n = text.length();
+		final int m = pattern.length();
+		while (i < n) {
+			if (text.charAt(i) == pattern.charAt(j)) {
+				i++;
+				j++;
+				if (j == m) {
+					occurrences.add(i - j);
+					j = lps[j - 1];
+				}
+			} else {
+				if (j != 0) {
+					j = lps[j - 1];
+				} else {
+					i++;
+				}
+			}
+		}
+		return occurrences;
+	}
+
+	// example runner
+	public static void main(String[] args) {
+		final String text = "AAAAABAAABA";
+		final String pattern = "AAAA";
+		final List<Integer> idx = search(text, pattern);
+		for (int pos : idx) {
+			System.out.println("Pattern starts: " + pos);
+		}
+	}
+}