Return the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack. (Single loop)

substring-searching-algo-02.cs

public int StrStr(string haystack, string needle) {
    if (needle == null || needle == "") return 0;
    if (needle.Length > haystack.Length) return - 1;
    if (needle.Length == haystack.Length) return haystack == needle ? 0 : -1;

    int j = 0;
    for (int i = 0; i < haystack.Length; i++) {
        
        if (j == 0 && haystack.Length - i < needle.Length) return -1;
        
        if (haystack[i] == needle[j]) {                
            j += 1;
            
            if (j == needle.Length) return i - j + 1;
        } else {
            i -= j;
            j = 0;
        }
    }

    return -1;
}