Longest Common Prefix

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

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+// geeksforgeeks explaination: https://www.geeksforgeeks.org/longest-common-prefix-using-sorting/
+/* The Longest Common Prefix (LCP) of a set of strings is the longest substring that appears at the beginning of each of the strings in the set. For example, given the strings:
+"flower"
+"flow"
+"flight"
+The longest common prefix is "fl", as it is the longest substring that is common at the start of all three strings. 
+Approach:-
+Sort the Array: Sort the array of strings to bring strings with common prefixes adjacent to each other.
+Identify Extremes: Select the first and last strings from the sorted array for comparison, as they will have the longest common prefix.
+Character Comparison: Compare the characters of the first and last strings until a mismatch is found, tracking the index of the last matching character.
+Return Prefix: Return the substring of the first string from the start to the index of the last matching character, which represents the longest common prefix.
+*/
+package com.thealgorithms.strings;
+import org.junit.Test;
+import static org.junit.Assert.assertEquals;
+import java.util.Arrays;
+
+public final class LongestCommonPrefix {
+    // Private constructor to prevent instantiation of utility class
+    private LongestCommonPrefix() {
+    }
+    // Method to find the longest common prefix
+    public static String longestPrefix(String[] str) {
+        int n = str.length;
+        if (n == 0) {
+            return "";
+        }
+        // Sort the array to bring similar prefixes closer
+        Arrays.sort(str);
+        // Compare the first and last strings after sorting
+        String first = str[0];
+        String last = str[n - 1];
+        int len = Math.min(first.length(), last.length());
+        // Find the longest common prefix
+        int i;
+        for (i = 0; i < len; i++) {
+            if (first.charAt(i) != last.charAt(i)) {
+                break;
+            }
+        }
+        return first.substring(0, i);
+    }
+    // JUnit Test cases
+    @Test
+    public void testLongestPrefix() {
+        // Test case 1
+        String[] input1 = { "flower", "flow", "flight" };
+        assertEquals("fl", longestPrefix(input1));
+
+        // Test case 2
+        String[] input2 = { "dog", "racecar", "car" };
+        assertEquals("", longestPrefix(input2));
+
+        // Test case 3
+        String[] input3 = {};
+        assertEquals("", longestPrefix(input3));
+
+        // Test case 4
+        String[] input4 = { "alone" };
+        assertEquals("alone", longestPrefix(input4));
+
+        // Test case 5
+        String[] input5 = { "same", "same", "same" };
+        assertEquals("same", longestPrefix(input5));
+
+        // Test case 6
+        String[] input6 = { "", "", "" };
+        assertEquals("", longestPrefix(input6));
+    }
+}
+/* 
+Time and Space Complexity:
+Time Complexity: O(n log n + m)
+- Sorting the array takes O(n log n), where n is the number of strings.
+- Comparing the first and last string takes O(m), where m is the length of the shortest string.
+- Overall, the time complexity is O(n log n + m).
+
+Space Complexity: O(n)
+- Sorting requires O(n) space for the array.
+- The space complexity for storing the prefix result is O(1), since it depends on the length of the prefix, which is part of the input.
+Therefore, the space complexity is O(n).
+*/