最长公共子串

1. 写一个算法查找两个字符串中公共最大的字符串。（要求：低时间复杂度）
如： abcdefabcd abefabghi 找出: efab

import java.util.Scanner;
import java.util.Stack;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        // 注意 hasNext 和 hasNextLine 的区别
        String str1 = in.next();
        String str2 = in.next();
        System.out.println(longestSubStr(str1, str2));
    }

    public static String longestSubStr(String str1, String str2) {
        String longestSubStr = "";
        String subStr = "";
        Boolean tag = false;
        for (int i = 0; i < str1.length(); ) {
            for (int j = 0; j < str2.length() && i < str1.length();) {
                if (str1.charAt(i) == str2.charAt(j)) {
                    subStr = subStr + str1.charAt(i);
                    i++;
                    j++;
                    tag = true;
                } else {
                    int k = 0;

                    if (tag) {
                        while (j < str2.length()) {
                            for (; k < subStr.length() && subStr.charAt(k) == str2.charAt(j);) {
                                k++;
                                j++;
                            }
                            if (k == subStr.length() && str1.charAt(i) == str2.charAt(j)) {
                                subStr = subStr + str2.charAt(j);
                                i++;
                                j++;
                            } else {
                                j++;
                            }
                        }
                    } else {
                        j++;
                    }
                }
            }
            if (subStr.length() > longestSubStr.length()) {
                longestSubStr = subStr;
            }
            if (!tag) {
                i++;
            }
            tag = false;
            subStr = "";
        }
        return longestSubStr;
    }
}