Z Algorithm

Author: Mrinal Chauhan

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

@@ -0,0 +1,61 @@
+package com.thealgorithms.strings;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Class to implement the Z Algorithm for pattern matching in strings.
+ * The Z Algorithm finds all occurrences of a pattern within a text.
+ */
+public final class Zalgorithm {
+    private Zalgorithm() {
+    }
+
+    /**
+     * Calculates the Z array for a given string.
+     *
+     * @param s the input string
+     * @return the Z array where Z[i] indicates the length of the longest substring
+     *         starting from s[i] that is also a prefix of s
+     */
+    public static int[] calculateZ(String s) {
+        int n = s.length();
+        int[] Z = new int[n];
+        int left = 0, right = 0;
+
+        for (int i = 1; i < n; i++) {
+            if (i <= right) {
+                Z[i] = Math.min(right - i + 1, Z[i - left]);
+            }
+            while (i + Z[i] < n && s.charAt(Z[i]) == s.charAt(i + Z[i])) {
+                Z[i]++;
+            }
+            if (i + Z[i] - 1 > right) {
+                left = i;
+                right = i + Z[i] - 1;
+            }
+        }
+        return Z;
+    }
+
+    /**
+     * Finds all occurrences of a pattern in the text using the Z Algorithm.
+     *
+     * @param text the text in which to search for the pattern
+     * @param pattern the pattern to search for
+     * @return a list of starting indices of occurrences of the pattern in the text
+     */
+    public static List<Integer> findPatternOccurrences(String text, String pattern) {
+        String combined = pattern + "$" + text;
+        int[] Z = calculateZ(combined);
+        List<Integer> occurrences = new ArrayList<>();
+        int patternLength = pattern.length();
+
+        for (int i = 0; i < Z.length; i++) {
+            if (Z[i] == patternLength) {
+                occurrences.add(i - patternLength - 1);
+            }
+        }
+        return occurrences;
+    }
+}