从一个字符串中，找出最长的对称字符串。即 xyzazyx 或者 xyzzyx。

find_longest_symmetric_string_fast.js

// 测试字符串
const stringArray = 'dadasdadawqfsdfaabbabcdedcbabbasadada13dfasafafa'.split(
  '');
// 测试字符串倒转
const stringArrayReverse = stringArray.slice().reverse();
const stringArrayReverseJoin = stringArrayReverse.join('');
// 最终结果
let resultString = '';
// 统计计算次数
let calcNum = 0;
const judgeFunc = num => {
  return stringArray.every((v, i, _) => {
    if (i + num > _.length) return false;
    calcNum++;
    const string = _.slice(i, i + num);
    const stringJoin = string.join('');
    if (
      stringArrayReverseJoin.indexOf(stringJoin) !== -1 &&
      resultString.length < stringJoin.length
    ) {
      resultString = stringJoin;
    }
    return true;
  });
};
console.time('test');
let i = stringArray.length;
for (; i > 2; i--) {
  if (resultString.length > i) break;
  judgeFunc(i);
}

console.log(`计算了：${calcNum}次，for 循环：${i} 次，结果为：${resultString}，长度：${resultString.length}`);
console.timeEnd('test');

find_longest_symmetric_string_low.js

// 测试字符串
const stringArray = 'dadasdadawqfsdfaabbabcdedcbabbasadada13dfasafafa'.split('');
// 最终结果
let resultString = '';
// 统计计算次数
let calcNum = 0;
// 主题函数
const judgeFunc = num => {
  return stringArray.every((v, i, _) => {
    if (i + num > _.length) return false;
    calcNum++;
    const string = _.slice(i, i + num);
    const stringJoin = string.join('');
    const halfArray = string.slice(0, string.length / 2 >> 0);
    const regExp = new RegExp(`${halfArray.join('')}.?${halfArray.reverse().join('')}`, 'g');
    if (regExp.test(stringJoin) && resultString.length < stringJoin.length) {
      resultString = stringJoin;
    }
    return true;
  });
};
console.time('test');
let i = stringArray.length;
// 使用逆向，节约计算资源
for (; i > 2; i--) {
  if (resultString.length > i) break;
  judgeFunc(i);
}
console.log(`计算了：${calcNum}次，for 循环：${i} 次，结果为：${resultString}，长度：${resultString.length}`);
console.timeEnd('test');