题目描述

枚举

样例

import java.util.*;
public class Main{
    /*
    枚举+双指针 会TLE
    static boolean check(int s){
        String b = Integer.toString(s) ;
        char[] a = b.toCharArray();
        int i = 0 ,j = a.length - 1;
        while(i <= j - 1){
            if(a[i] != a[j]){
                return false;
            }
            i++;
            j--;
        }
        return true;
    }

    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int s = sc.nextInt();
        int e = sc.nextInt();
        int res = 0;
        for(int i = s; i <= e; i++){
            if(check(i)) res++;
        }
        System.out.println(res);
    }
    */

    /*
      回文串左右对称，因此可以只枚举左半边，这样只需枚举 0∼9999总共一万个数，然后判断：
         1.整个八位数构成的日期是否合法；
         2.是否在范围内
    */
    static int[] months = {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

    //%取后几位数 /就是去前几位数
    static boolean check(int date) {
        int year = date / 10000;
        int month = date % 10000 / 100;
        int day = date % 100;

        if (month == 0 || month >= 13 || day == 0) return false;

        if (month != 2 && day > months[month]) return false;
        if (month == 2) {
            boolean leap = (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0);
            if (day > 28 + (leap ? 1 : 0)) return false;
        }

        return true;
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int date1 = scanner.nextInt();
        int date2 = scanner.nextInt();

        int res = 0;
        for (int i = 0; i < 10000; i++) {
            //x和r都等于r
            int x = i, r = i;
            //通过循环4次，拼凑成完整的八位数
            for (int j = 0; j < 4; j++) {
                //r先乘于10，再加上x的最后一位
                r = r * 10 + x % 10;
                //去除x的最后一位
                x /= 10;
            }

            //判断日期是否合法
            if (r >= date1 && r <= date2 && check(r)) {
                res++;
            }
        }

        System.out.println(res);
    }
}