strStr

class Solution {
public:
    void get_nextval(const char *T, int next[])
    {
       // 求模式串T的next函数值并存入数组 next。
       int j = 0, k = -1;
       next[0] = -1;
       while ( T[j] != '\0' )
       {
              if (k == -1 || T[j] == T[k])
              {
                     ++j; ++k;
                     if (T[j]!=T[k])
                            next[j] = k;
                     else
                            next[j] = next[k];
              }// if
              else
                     k = next[k];
       }// while
   
    }// get_nextval　
    char *strStr(char *haystack, char *needle) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if( !needle || *needle == '\0' ) return haystack;
        if( !haystack || *haystack =='\0'  )  return NULL;
        int len=0;
        const char * c=needle;
        while(*c++!='\0')//移动指针比移动下标快。
        {    
              ++len;//字符串长度。
        }
        int *next=new int[len+1];
        get_nextval(needle,next);
        int index=0,i=0,j=0;
       while(haystack[i]!='\0'  && needle[j]!='\0' )
       {
              if(haystack[i]== needle[j])
              {
                     ++i;// 继续比较后继字符
                     ++j;
              }
              else
              {
                     index += j-next[j];
                     if(next[j]!=-1)
                            j=next[j];// 模式串向右移动
                     else
                     {
                            j=0;
                            ++i;
                     }
              }
       }//while
   
       delete []next;
       if(needle[j]=='\0')
              return haystack + index;// 匹配成功
       else
              return NULL;      
    }
};