[28] 找出字符串中第一个匹配项的下标

/**
 * @param {string} haystack
 * @param {string} needle
 * @return {number}
 */
var strStr = function (haystack, needle) {
  if (haystack.length == 0 || needle.length == 0) return -1;
  let nextArray = new Array(needle.length);
  nextArray[0] = 0;

  for (let i = 0, j = 1; j < needle.length; j++) {
    while (i > 0 && needle[i] != needle[j]) i = nextArray[i - 1];
    if (needle[i] == needle[j]) i++;
    nextArray[j] = i;
  }
  for (let i = 0, j = 0; i < haystack.length; i++) {
    while (j > 0 && haystack[i] != needle[j]) j = nextArray[j - 1];
    if (haystack[i] == needle[j]) j++
    if (j == needle.length) return i + 1 - j;
  }
  return -1;
};