Answer 1

搜索了一下KMP算法：
https://zh.wikipedia.org/wiki/%E5%85%8B%E5%8A%AA%E6%96%AF-%E8%8E%AB%E9%87%8C%E6%96%AF-%E6%99%AE%E6%8B%89%E7%89%B9%E7%AE%97%E6%B3%95

// 应该...没有bug吧^^
function getIndex (S, T) {
  const n = S.length;
  const m = T.length;
  if (n < m) {
    return -1;
  }
  if (n === m) {
    return S === T ? 0 : -1;
  }

  const start = T[0];
  const end = T[m - 1];
  let i = 0;

  while (i < n) {
    if (n - i < m) {
      return -1;
    }

    const currS = S[i];
    const currSEnd = S[i + m - 1];
    let mathing = true;

    if (currS !== start || currSEnd !== end) {
      i++;
      continue;
    }

    let j = 0;
    let step = 0;

    while (j < m) {
      const currS = S[i + j];
      const currSEnd = S[i + j + m - 1];
      const isEnd = j === m - 1;

      if (mathing) {
        const currW = T[j];
        mathing = currS === currW;

        if (mathing && isEnd) {
          return i
        }
      }
      
      if (!mathing && step !== 0) {
        break;
      }

      if (j > 0 && step === 0) {
        if (isEnd) {
          step = m;
        }
        if (currS === start && currSEnd === end) {
          step = j;
        }
      }
      j++;
    }
    i += step || 1;
  }

  return -1;
}