穷举法破解密码(递归 + 多层for循环)

题目要求：

1. 生成一个长度是3的随机字符串，把这个字符串作为当做密码
2. 使用穷举法生成长度是3个字符串，匹配上述生成的密码
要求： 分别使用多层for循环 和 递归解决上述问题

1）首先用递归来实现

public static void guessNum(char[] c, int index, String password){
        boolean flag = false;
        if (flag) {
            return;
        }
        for (int i = '0'; i <= 'z'; i++) {
            if (!Character.isLetterOrDigit(i)) {
                continue;
            }
            c[index] = (char) i;
            if (index == c.length - 1) {
                String guess = new String(c);
                if (guess.equals(password)) {
                    System.out.println("找到了密码" + guess);
                    flag = true;
                    return;
                }
            }
            else {
                guessNum(c, index + 1, password);
            }

        }
    }

    public static void main(String[] args) {
        char[] c = new char[3];

        int start = '0';
        int end = 'z' + 1;

        for (int i = 0; i < c.length; i++) {
            while (true) {
                char c1 = (char) (Math.random() * (end - start) + start);
                if (Character.isLetter(c1) || Character.isDigit(c1)) {
                    c[i] = c1;
                    break;
                }
            }
        }
        String str1 = new String(c);
        System.out.println("随机生成的字符串是：" + str1);
        System.out.println("----------");
        guessNum(c, 0, str1);

2）下面用多层for循环实现

public static void main(String[] args) {
        char[] c = new char[3];

        int start = '0';
        int end = 'z' + 1;

        for (int i = 0; i < c.length; i++) {
            while (true) {
                char c1 = (char) (Math.random() * (end - start) + start);
                if (Character.isLetter(c1) || Character.isDigit(c1)) {
                    c[i] = c1;
                    break;
                }
            }
        }
        String str1 = new String(c);
        System.out.println("随机生成的字符串是：" + str1);
        System.out.println("----------");
        String result = "";
        char[] g = new char[3];

        for (int i = '0'; i <= '9'; i++) {
            result += (char) i;
        }
        for (int i = 'a'; i <= 'z'; i++) {
            result += (char) i;
        }
        for (int i = 'A'; i <= 'Z'; i++) {
            result += (char) i;
        }

        for (int i = 0; i < result.length(); i++){
            for (int j = 0; j < result.length(); j++){
                for (int k = 0; k < result.length(); k++){
                    g[0] = result.charAt(i);
                    g[1] = result.charAt(j);
                    g[2] = result.charAt(k);
                    String guess = new String(g);
                    if (str1.equals(guess)) {
                        System.out.println("找到了密码" + guess);
                    }
                }
            }
        }


    }