5. 最长回文子串

/**
 * @param {string} s
 * @return {string}
 */
var longestPalindrome = function (s) {
  let max = ''
  if (s.length < 2) {
    return s
  }

  /**
   * 时间复杂度高，需优化
   * const isPalindrome = (str) => {
   *  return str === str.split('').reverse().join('')
   * }
   */

  const isPalindrome = (str) => {
    if (str.length === 1) {
      return true
    }
    let i = 0,
      j = str.length - 1
    while (i < j) {
      if (str[i] !== str[j]) {
        return false
      }
      i++
      j--
    }
    return true
  }
  const len = s.length
  for (let i = 0; i < len; i++) {
    let char = s[i]
    for (let j = len - 1; j >= i + max.length; j--) {
      if (char === s[j]) {
        const subStr = s.slice(i, j + 1)
        if (isPalindrome(subStr) && subStr.length > max.length) {
          max = subStr
        }
      }
    }
  }
  return max
}