时间复杂度：

public int strStr(String haystack, String needle) {
        int[] next = new int[needle.length()];
        //next数组的生成
        next[0] = 0;
        int prefixLen = 0;//共同前后缀长度
        int i = 1, j = 1;//i,j复用
        while (i < needle.length()) {
            if (needle.charAt(prefixLen) == needle.charAt(i)) {
                prefixLen++;
                next[j++] = prefixLen;
                i++;
            } else {
                if (prefixLen == 0) {
                    next[j++] = 0;
                    i++;
                } else {
                    prefixLen = next[prefixLen - 1];
                }
            }
        }

        i = j = 0;
        while (i < haystack.length()) {
            if (haystack.charAt(i) == needle.charAt(j)) {
                i++;
                j++;
            } else if (j > 0) {
                j = next[j - 1];
            } else {
                i++;
            }
            if (j == needle.length())
                return i - j;
        }
        return -1;
    }