实现一个回文算法

function palindrome(str: string): Array<[number, number]> {
  const palindromeIndex: Array<[number, number]> = [];
  const strLength: number = str.length;

  if (strLength < 3) {
    return palindromeIndex;
  }

  for (let i: number = 1; i < strLength - 1; i++) {
    // 奇数
    let startIndex: number | null = null,
      endIndex: number | null = null;

    for (let k: number = i - 1, m: number = i + 1; k >= 0 && m < strLength; k--, m++) {
      if (str[k] === str[m]) {
        [startIndex, endIndex] = [k, m];
      } else {
        break;
      }
    }

    if (startIndex !== null && endIndex !== null) {
      palindromeIndex.push([startIndex, endIndex]);
    }

    // 偶数
    [startIndex, endIndex] = [null, null];

    for (let p: number = i, t: number = i + 1; p >= 0 && t < strLength; p--, t++) {
      if (str[p] === str[t]) {
        [startIndex, endIndex] = [p, t];
      } else {
        break;
      }
    }

    if (startIndex !== null && endIndex !== null) {
      palindromeIndex.push([startIndex, endIndex]);
    }
  }

  return palindromeIndex;
}