Add a recursive version of the Longest Common Substring.

Author: trekhleb

src/algorithms/string/longest-common-substring/__test__/longestCommonSubstring.test.js

@@ -7,6 +7,8 @@ describe('longestCommonSubstring', () => {
     expect(longestCommonSubstring('', 'ABC')).toBe('');
     expect(longestCommonSubstring('ABABC', 'BABCA')).toBe('BABC');
     expect(longestCommonSubstring('BABCA', 'ABCBA')).toBe('ABC');
+    expect(longestCommonSubstring('sea', 'eat')).toBe('ea');
+    expect(longestCommonSubstring('algorithms', 'rithm')).toBe('rithm');
     expect(longestCommonSubstring(
       'Algorithms and data structures implemented in JavaScript',
       'Here you may find Algorithms and data structures that are implemented in JavaScript',

src/algorithms/string/longest-common-substring/__test__/longestCommonSubstringRecursive.test.js

@@ -0,0 +1,17 @@
+import longestCommonSubstring from '../longestCommonSubstringRecursive';
+
+describe('longestCommonSubstringRecursive', () => {
+  it('should find longest common substring between two strings', () => {
+    expect(longestCommonSubstring('', '')).toBe('');
+    expect(longestCommonSubstring('ABC', '')).toBe('');
+    expect(longestCommonSubstring('', 'ABC')).toBe('');
+    expect(longestCommonSubstring('ABABC', 'BABCA')).toBe('BABC');
+    expect(longestCommonSubstring('BABCA', 'ABCBA')).toBe('ABCA');
+    expect(longestCommonSubstring('sea', 'eat')).toBe('ea');
+    expect(longestCommonSubstring('algorithms', 'rithm')).toBe('rithm');
+    expect(longestCommonSubstring(
+      'Algorithms and data structures implemented in JavaScript',
+      'Here you may find Algorithms and data structures that are implemented in JavaScript',
+    )).toBe('Algorithms and data structures implemented in JavaScript');
+  });
+});

src/algorithms/string/longest-common-substring/longestCommonSubstring.js

@@ -1,4 +1,6 @@
 /**
+ * Longest Common Substring (LCS) (Dynamic Programming Approach).
+ *
  * @param {string} string1
  * @param {string} string2
  * @return {string}

src/algorithms/string/longest-common-substring/longestCommonSubstringRecursive.js

@@ -0,0 +1,36 @@
+/* eslint-disable no-param-reassign */
+/**
+ * Longest Common Substring (LCS) (Recursive Approach).
+ *
+ * @param {string} string1
+ * @param {string} string2
+ * @return {number}
+ */
+export default function longestCommonSubstringRecursive(string1, string2) {
+  /**
+   *
+   * @param {string} s1
+   * @param {string} s2
+   * @return {string} - returns the LCS (Longest Common Substring)
+   */
+  const lcs = (s1, s2, memo = {}) => {
+    if (!s1 || !s2) return '';
+
+    if (memo[`${s1}:${s2}`]) return memo[`${s1}:${s2}`];
+
+    if (s1[0] === s2[0]) {
+      return s1[0] + lcs(s1.substring(1), s2.substring(1), memo);
+    }
+
+    const nextLcs1 = lcs(s1.substring(1), s2, memo);
+    const nextLcs2 = lcs(s1, s2.substring(1), memo);
+
+    const nextLongest = nextLcs1.length >= nextLcs2.length ? nextLcs1 : nextLcs2;
+
+    memo[`${s1}:${s2}`] = nextLongest;
+
+    return nextLongest;
+  };
+
+  return lcs(string1, string2);
+}