class Solution {

public:

    bool isEqual(char *a,char *b)

    {

        char* chPointer = a;

        char* chPointer2 = b;

        while(*chPointer2!= '\0' && *chPointer!='\0' )

        {

            if(*chPointer == *chPointer2)

            {

                chPointer++;

                chPointer2++;

            }

            else

                return false;

        }

        return true;

    }

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

        char* chPointer = haystack;

        char* chPointer2 = needle;

        if(haystack == NULL || needle ==NULL || haystack =="")

            return NULL;

        while(chPointer)

        {

            if(isEqual(chPointer,needle))

                return chPointer;

            chPointer++;

        }

        return NULL;

    }

};

class Solution {

public:

    void compute_prefix(const char* pattern,int next[])

    {

        int i;

        int j = -;

        const int m = strlen(pattern);

        next[] = j;

        for(i =;i<m;i++)

        {

            while(j>- && pattern[j+] != pattern[i]) j = next[j];

            if(pattern[i]== pattern[j+])

                j++;

            next[i] = j;

        }

    }

    int kmp(const char* text , const char* pattern)

    {

        int i;

        int j = -;

        const int n = strlen(text);

        const int m = strlen(pattern);

        if(n == m && m==)

            return ;

        if(m == )

            return ;

        int *next = (int *)malloc(sizeof(int) * m);

        compute_prefix(pattern, next);

        for(i = ;i <n;i++)

        {

            while(j >- && pattern[j+] != text[i])

                j = next[j];

            if(text[i] == pattern[j+]) j++;

            if(j==m-)

            {

                free(next);

                return i-j;

            }

        }

    }

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

       int position = kmp(haystack,needle);

       if(position == -)

           return NULL;

       else

           return (char*)haystack + position;

    }

};