LeetCode_implement strstr ()

 Implement strStr().



Returns a pointer to the first occurrence of needle in haystack, or null if needle is not part of haystack.

class Solution {

public:

    void calculatNext(char *pattern)

    {

       

       int i = 0, k = -1;

       next[0] = -1;

       while( i < sizeNeed -1) // next[i+1]       

       {

       

         while(k >= 0 && pattern[i] != pattern[k]) k = next[k];

       

         i++; k++;

         

         next[i] = pattern[i] == pattern[k] ? next[k] : k;

        

       }

    }

    char *strStr(char *haystack, char *needle) {

        // Start typing your C/C++ solution below

        // DO NOT write int main() function

        sizeHay = strlen(haystack);

        sizeNeed = strlen(needle);

        

        if(sizeNeed == 0)  return haystack;

        next.resize(sizeNeed);

        

        calculatNext(needle);

        

        int i =  0, j = 0;

        

        while(i< sizeHay && j < sizeNeed)

        {

            if(j == -1 || haystack[i] == needle[j]){           

              i++;j++;

            }else

              j = next[j];            

        }

        

        if(j >= sizeNeed)

          return haystack+(i - j);

         else

          return NULL ; 

    }

    

private :

  int sizeHay;

  int sizeNeed ;

  vector<int> next ;

};