# 28.implement strstr

**<font color=red>难度Easy</font>**

## 刷题内容

> 原题连接

* https://leetcode.com/problems/implement-strstr/
> 内容描述

```
Implement strStr().

Return the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

Example 1:

Input: haystack = "hello", needle = "ll"
Output: 2
Example 2:

Input: haystack = "aaaaa", needle = "bba"
Output: -1
Clarification:

What should we return when needle is an empty string? This is a great question to ask during an interview.

For the purpose of this problem, we will return 0 when needle is an empty string. This is consistent to C's strstr() and Java's indexOf().
```
> 思路
******- 时间复杂度: O(N)******- 空间复杂度: O(1)******

这题直接遍历 haystack ，匹配到与 needle 第一个字符相等时，就遍历 needle，挨个比较字符。

```cpp
class Solution {
public:
    int strStr(string haystack, string needle) {
        int j = 0,i = 0,index= 0;
        while(i < haystack.size() && j < needle.size())
        {
            if(haystack[i] == needle[j])
            {
                if(!j)
                    index = i;
                j++;
                i++;
            }
            else
            {
                i = ++index;
                j = 0;
            }
        }
        if(j == needle.size())
            return index;
        return -1;
    }
};
```