实现 strStr

function strStr(haystack: string, needle: string): number {
  // tail 实际为 head(0) + needle.length + 1
  let head = 0, tail = needle.length - 1

  while (tail < haystack.length) {
    if (haystack.substr(head, needle.length) === needle) return head
    head++
    tail++
  }

  return -1
}