题解 | 自守数

import java.util.ArrayList;
import java.util.Scanner;

// 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);
        ArrayList<Character> list = new ArrayList<>();
        // 注意 hasNext 和 hasNextLine 的区别
        int n = in.nextInt();
        int count = 0, len = 0;
        //求n的位数
        for (int i = 0; i <= n; i++) {
            len = Integer.toString(i).length();
            if (len == 1 && i * i == i) {
                count++;
            } else {
                StringBuilder builder = new StringBuilder();
                for (char c : Integer.toString(i * i).toCharArray()) {
                    list.add(c);
                }
                for (int j = len; j > 0; j--) {
                    builder.append(list.get(list.size() - j));
                }
                if (builder.toString().equals(Integer.toString(i))) {
                    count++;
                }
            }
        }
        System.out.println(count);
    }
}