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.

　　KMP :

class Solution {
public:
    void calculatNext(char *pattern)
    {
       
       int i = 0, k = -1;
       next[0] = -1;
       while( i < sizeNeed -1) //计算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 ;
};