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problems/0028.实现strStr().md

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+
+## 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;
+    }
+};
+
+
+```
+> 笔者在先后在腾讯和百度从事技术研发多年，利用工作之余重刷leetcode，本文  [GitHub](https://github.com/youngyangyang04/leetcode-master )：https://github.com/youngyangyang04/leetcode-master 已经收录，欢迎star，fork，共同学习，一起进步。