feat(string): 添加回文串递归算法

Author: hushicai

src/algorithms/string/is-palindrome/README.md

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+## 回文串
+
+递归算法

src/algorithms/string/is-palindrome/__tests__/isPalindrome.test.js

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+import isPalindrome from '../isPalindrome';
+
+describe('isPalindrome', () => {
+  it('should be a palindrome string', () => {
+    expect(isPalindrome('level')).toBe(true);
+    expect(isPalindrome('noon')).toBe(true);
+  });
+  it('should not be a palindrome string', () => {
+    expect(isPalindrome('devel')).toBe(false);
+  });
+});

src/algorithms/string/is-palindrome/isPalindrome.js

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+export default function isPalindrome(str) {
+  const len = str.length;
+  const first = str[0];
+  const last = str[str.length - 1];
+
+  if (len === 0 || len === 1) {
+    return true;
+  }
+
+  if (first === last) {
+    return isPalindrome(str.substring(1, str.length - 1));
+  }
+  return false;
+}