## 题目地址 
https://leetcode-cn.com/problems/implement-strstr/

## 思路 


## C++代码

```
class Solution {
public:
    void preKmp(int* next, const string& s){
        next[0]=-1;
        int j=-1;
        for(int i=1;i<s.size();i++){
            while(j>=0 && s[i]!=s[j+1])
                j = next[j];
            if(s[i]==s[j+1])
                j++;
            next[i]=j;
        }
    }
    int strStr(string haystack, string needle) {
        if (needle.size() == 0) {
            return 0;
        }
        int next[needle.size()];
        preKmp(next, needle);

        int j = -1;
        for (int i = 0; i < haystack.size(); i++) {
            while(j >= 0 && haystack[i] != needle[j + 1]) {
                j = next[j];
            }
            if (haystack[i] == needle[j + 1]) {
                j++;
            }
            if (j == (needle.size() -1) ) {
                return (i - needle.size() + 1);
            }
        }
        return -1;
    }
};


```
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