Implement strStr()

Implement strStr().

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

又一次难点在于看不懂题。其实就是求一个函数判断needle是否是haystack的子字符串，如果是返回起始元素的index，否则返回-1

最优化解就是KMP算法 https://en.wikipedia.org/wiki/Knuth%E2%80%93Morris%E2%80%93Pratt_algorithm

下面是一个naive解法

public int strStr(String haystack, String needle) {
    if (haystack == null || needle == null) {
        return 0;
    }
    if (needle.length() == 0) {
        return 0;
    }

    for (int i = 0; i < haystack.length(); i++) {
        if (i + needle.length() > haystack.length()) {
            return -1;
        }
        int m = i;
        for (int j = 0; j < needle.length(); j++) {
            if (haystack.charAt(m++) == needle.charAt(j)) {
                if (j == needle.length() -1) {
                    return i;
                }
            } else {
                break;
            }
        }
    }
    return -1;
}