add: 最长回文字符串

test/string/longest-palindromic-substring.test.js

@@ -0,0 +1,6 @@
+import { longestPalindrome } from '../../src/string/longest-palindromic-substring'
+
+test('最长回文字符串', () => {
+  expect(longestPalindrome('babad')).toBe('aba') // bab也是有效答案
+  expect(longestPalindrome('cbbd')).toBe('bb')
+})