[LeetCode] 28. Implement strStr()

Implement strStr().

Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

int* preprocess(char *needle)
{
    int len = strlen(needle);
    int *p = (int *)malloc(sizeof(int) * len);
    int j = -1;
    
    for (int i = 0; i < len; i++){
        p[i] = -1;
    }

    for (int i = 1; i < len; i++){
        while ((j >= 0) && (needle[j+1] != needle[i])) j = p[j];
        if (needle[j + 1] == needle[i]) j++;
        p[i] = j;
    }

    return p;
}

int strStr(char* haystack, char* needle) {
    char *ph = haystack;
    char* pn = needle;
    char* pstart = NULL;
  
    if (!*needle) return 0;
    if (!*haystack) return -1;   

    int* p = preprocess(needle);
    
    int j = -1;
    for (int i = 0; i < strlen(haystack); i++){
        while ((j >= 0) && (needle[j+1] != haystack[i])) j = p[j];
        if (needle[j + 1] == haystack[i]) j++;
        if ((j + 1) == strlen(needle))
           return (i - strlen(needle) + 1); 
    }

    return -1;
}