func strStr(haystack string, needle string) int {
    if len(haystack) < len(needle) {
        return -1
    }
    var i, j int
    for i < len(haystack) {
        if j >= len(needle) {
            return i - j
        }
        if haystack[i] == needle[j] {
            j++
            i++
        } else {
            i = i - j + 1
            j = 0
        }
    }

    if j == len(needle) {
        return i - j
    }
    return -1
}