字符串正序查找子串

#include <stdio.h>
#include <string.h>

char *myStrstr(const char *haystack, const char *needle);

int
main(void)
{
    char *s = "Hello world";
    char *t = "ll";
    char *r = myStrstr(s, t);
    printf(r);

    return 0;
}

char *
myStrstr(const char *haystack, const char *needle)
{
    if (NULL == haystack || NULL == needle)
        return NULL;

    size_t i;
    size_t hay_len, need_len;
    const char *p = haystack;

    hay_len = strlen(haystack);
    need_len = strlen(needle);

    if (need_len == 0)
        return (char *)haystack;

    if (hay_len < need_len)
        return NULL;

    while (p <= haystack + hay_len - need_len) {
        for (i = 0; i < need_len; i++)
            if (p[i] != needle[i])
                goto next;
        return (char *)p;

        next:
            p++;
    }

    return NULL;
}