# 找出字符串中第一个匹配项的下标

题目链接: <https://leetcode.cn/problems/find-the-index-of-the-first-occurrence-in-a-string>

## 解题思路：

1. 枚举原串 `haystack` 中的每个字符作为发起点，每次从原串的发起点和匹配串的首位开始尝试匹配：
   * 匹配成功：返回本次匹配的原串发起点
   * 匹配失败：枚举原串的下一个发起点，重新尝试匹配

```go
func strStr(haystack string, needle string) int {
	for i := 0; i < len(haystack)-len(needle)+1; i++ {
		if haystack[i] == needle[0] && haystack[i:i+len(needle)] == needle {
			return i
		}
	}

	return -1
}
```

## 复杂度分析

* **时间复杂度：** 时间复杂度是 $$O(mn)$$，其中 $$m$$ 是字符串 `haystack` 的长度，$$n$$ 是字符串 `needle` 的长度。在函数中，需要遍历字符串 `haystack` 一次，并且对于每个字符，需要比较与子串 `needle` 是否相等
* **空间复杂度：** 只使用了常数个变量，因此空间复杂度为 $$O(1)$$


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://leetcodebook-1.gitbook.io/top-interview-150/array-string/find-the-index-of-the-first-occurrence-in-a-string.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.