最大相同子串问题

public String longestCommonStr(String str1, String str2) {
    int len1 = str1.length();
    int len2 = str2.length();
    String res = "";
    if (len1 < len2) {//默认str1比str2长
        String temp = str1;
        str1 = str2;
        str2 = temp;
    }
    label:for (int len = str2.length(); len > 0 ; len--) {
        for (int i = 0; i + len < str2.length(); i++) {
            String subStr = str2.substring(i,i + len);
            if (str1.contains(subStr)) {
                res = subStr;
                break label;
            }
        }
    }
    return res;
}