add: 最长回文字符串

.vscode/settings.json

@@ -21,6 +21,7 @@
     "nums",
     "powx",
     "pwwkew",
+    "qian",
     "subarray",
     "subarrays",
     "umghlrlose",

README.md

@@ -95,6 +95,11 @@ LeetCode 与 LintCode 解题记录。此为个人练习仓库，代码中对重
 
   - LeetCode 1108. IP 地址无效化 <https://leetcode-cn.com/problems/defanging-an-ip-address/>
 
+- [最长回文子串](src/string/longest-palindromic-substring.js)
+
+  - LeetCode 5. 最长回文子串 <https://leetcode-cn.com/problems/longest-palindromic-substring/>
+  - LintCode 200. 最长回文子串 <https://www.lintcode.com/problem/longest-palindromic-substring/>
+
 - [每个元音包含偶数次的最长子字符串](src/string/find-the-longest-substring-containing-vowels-in-even-counts.js)
 
   - LeetCode 1371. 每个元音包含偶数次的最长子字符串 <https://leetcode-cn.com/problems/find-the-longest-substring-containing-vowels-in-even-counts/>

src/string/longest-palindromic-substring.js

@@ -0,0 +1,36 @@
+export const longestPalindrome = function (s) {
+  if (!s || !s.trim()) return ''
+  if (s.length === 1) return s
+  if (s.length === 2) return s[0] === s[1] ? s[0] + s[1] : s[1]
+  let result = ''
+  /**
+  *扩散坐标
+  */
+  const calPalindromeIndex = function (left, right, s) {
+    const len = s.length
+    while (left >= 0 && right < len && s[left] === s[right]) {
+      left--
+      right++
+    }
+    return { left: left + 1, right: right }
+  }
+  for (let i = 0, len = s.length; i < len; i++) {
+    let even = ''
+    let odd = ''
+    if (s[i] === s[i + 1]) {
+      // 经过当前位与下一位判断已构成回文，扩散位直接从下一位开始，可以提速
+      const evenIndex = calPalindromeIndex(i - 1, i + 2, s)
+      even = s.slice(evenIndex.left, evenIndex.right)
+    }
+    const oddIndex = calPalindromeIndex(i - 1, i + 1, s)
+    odd = s.slice(oddIndex.left, oddIndex.right)
+    const re = odd.length > even.length ? odd : even
+    result = result.length > re.length ? result : re
+  }
+  return result
+}
+
+// 作者：liu-zi-qian-2
+// 链接：https://leetcode-cn.com/problems/longest-palindromic-substring/solution/5-zui-chang-hui-wen-zi-chuan-by-liu-zi-qian-2/
+// 来源：力扣（LeetCode）
+// 著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。

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')
+})