28.Find the Index of the First Occurrence in a String
===

###### tags: `Medium`,`Two Pointers`,`String`

[28. Find the Index of the First Occurrence in a String](https://leetcode.com/problems/find-the-index-of-the-first-occurrence-in-a-string/)

### 題目描述

Given two strings `needle` and `haystack`, return the index of the first occurrence of `needle` in `haystack`, or `-1` if `needle` is not part of `haystack`.

### 範例

**Example 1:**

Input: haystack = "sadbutsad", needle = "sad"
Output: 0
Explanation: "sad" occurs at index 0 and 6.
The first occurrence is at index 0, so we return 0.

**Example 2:**

Input: haystack = "leetcode", needle = "leeto"
Output: -1
Explanation: "leeto" did not occur in "leetcode", so we return -1.

**Constraints**:

* 1 <= `haystack.length`, `needle.length` <= 10^4^
* `haystack` and `needle` consist of only lowercase English characters.

### 解答

#### Javascript

function strStr(haystack, needle) {
  for (let i = 0; i < haystack.length; i++) {
    if (haystack[i] === needle[0]) {
      if (needle.length === 1) return i;
      for (let j = 1; j < needle.length; j++) {
        if (haystack[i + j] !== needle[j]) break;
        if (j === needle.length - 1) return i;
      }
    }
  }

  return -1;
}

function strStr(haystack, needle) {
  return haystack.indexOf(needle);
}

> Time: $O(mn)$
> 暴力比對也會過，好神秘，還以為一定要寫個KMP什麼的...
> [name=Marsgoat][time=Mar 3, 2023]

### Reference

https://mp.weixin.qq.com/s/eLtQABruL9-Aut5fXl5x-g
分享一下關於indexOf的底層實作


> 天宇大神表示 : 88 stupid company

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