feat(dp): Longest Palindromic Substring

README.md

@@ -126,6 +126,7 @@ A collection of JavaScript problems and solutions for studying algorithms.
 ### Dynamic Programming
 
 - [Longest Increasing Subsequence](src/dynamic-programming/longest-increasing-subsequence.js)
+- [Longest Palindromic Substring](src/dynamic-programming/longest-palindromic-substring.js)
 
 ## Author
 

src/dynamic-programming/__tests__/longest-palindromic-substring-test.js

@@ -0,0 +1,22 @@
+import { assert } from 'chai';
+import { longestPalindrome, longestPalindromeExpand } from '../longest-palindromic-substring';
+
+describe('Longest Palindromic Substring', () => {
+  const testCases = [['babad', ['aba', 'bab']], ['aaaa', ['aaaa']]];
+
+  testCases.forEach((testCase, index) => {
+    it(`should get the longest palindromic substring using dynamic programming`, () => {
+      const s = testCase[0];
+      const expected = testCase[1];
+      const actual = longestPalindrome(s);
+      assert.include(expected, actual);
+    });
+
+    it(`should get the longest palindromic substring using expanding around center solution`, () => {
+      const s = testCase[0];
+      const expected = testCase[1];
+      const actual = longestPalindromeExpand(s);
+      assert.include(expected, actual);
+    });
+  });
+});

src/dynamic-programming/longest-palindromic-substring.js

@@ -0,0 +1,85 @@
+/**
+ * Longest Palindromic Substring
+ *
+ * Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000.
+ *
+ * Example:
+ *
+ * Input: "babad"
+ *
+ * Output: "bab"
+ *
+ * Note: "aba" is also a valid answer.
+ * Example:
+ *
+ * Input: "cbbd"
+ *
+ * Output: "bb"
+ *
+ */
+
+/**
+ * Solution 1: Dynamic Programming
+ *
+ * @param {string} s
+ * @return {string}
+ */
+const longestPalindrome = s => {
+  const n = s.length;
+
+  const dp = [];
+  for (let i = 0; i < n; i++) {
+    dp[i] = [];
+  }
+
+  let result = '';
+  for (let i = n - 1; i >= 0; i--) {
+    for (let j = i; j < n; j++) {
+      dp[i][j] = s[i] === s[j] && (j - i <= 2 || dp[i + 1][j - 1]);
+
+      if (dp[i][j] && j - i + 1 > result.length) {
+        result = s.substring(i, j + 1);
+      }
+    }
+  }
+
+  return result;
+};
+
+/**
+ * Solution 2: Expand Around Center
+ *
+ * @param {string} s
+ * @return {string}
+ */
+const longestPalindromeExpand = s => {
+  let start = 0;
+  let end = 0;
+
+  for (let i = 0; i < s.length; i++) {
+    const len1 = expandAroundCenter(s, i, i);
+    const len2 = expandAroundCenter(s, i, i + 1);
+    const len = Math.max(len1, len2);
+
+    if (len > end - start) {
+      start = i - Math.floor((len - 1) / 2);
+      end = i + Math.floor(len / 2);
+    }
+  }
+
+  return s.substring(start, end + 1);
+};
+
+const expandAroundCenter = (s, left, right) => {
+  let L = left;
+  let R = right;
+
+  while (L >= 0 && R < s.length && s[L] == s[R]) {
+    L--;
+    R++;
+  }
+
+  return R - L - 1;
+};
+
+export { longestPalindrome, longestPalindromeExpand };