题解 | #幸运数字#

#include <iostream>
#include <string>
using namespace std;

int a, b, count = 0;

// 计算数字的位数
int intlen(int num) {
    int i = 0;
    while(num) {
        num /= 10;
        i++;
    }
    return i;
}

// 递归生成数字
void generate(char cs[], int index, int len) {
    // 超过10位直接返回
    if(len >= 10) return;
    
    // 当前位数已满，检查数字是否在范围内
    if(index == len) {
        int num = atoi(cs);
        if(num >= a && num <= b) {
            count++;
        }
        return;
    }
    
    // 在当前位置尝试6和8
    for(int i = 0; i < 2; i++) {
        cs[index] = (i == 0) ? '6' : '8';
        generate(cs, index + 1, len);
    }
}

int main() {
    // 输入并检查合法性
    cin >> a;
    if(a < 1 || a > 1000000000) {
        cout << "-1" << endl;
        return 0;
    }
    
    cin >> b;
    if(b < a || b > 1000000000) {
        cout << "-1" << endl;
        return 0;
    }
    
    // 计算位数范围
    int lenA = intlen(a);
    int lenB = intlen(b);
    
    // 对每个可能的位数生成组合
    for(int i = lenA; i <= lenB; i++) {
        char cs[11] = {0};  // 多分配一位用于字符串结束符
        generate(cs, 0, i);
    }
    
    cout << count << endl;
    return 0;
}

import java.util.*;

public class Main {
    static int count = 0;
    static int a, b;
    
    // 计算数字的位数
    static int intlen(int num) {
        int i = 0;
        while(num > 0) {
            num /= 10;
            i++;
        }
        return i;
    }
    
    // 递归生成数字
    static void generate(char[] cs, int index, int len) {
        if(len >= 10) return;
        
        if(index == len) {
            int num = Integer.parseInt(new String(cs, 0, len));
            if(num >= a && num <= b) {
                count++;
            }
            return;
        }
        
        for(int i = 0; i < 2; i++) {
            cs[index] = (i == 0) ? '6' : '8';
            generate(cs, index + 1, len);
        }
    }
    
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        
        a = sc.nextInt();
        if(a < 1 || a > 1000000000) {
            System.out.println("-1");
            return;
        }
        
        b = sc.nextInt();
        if(b < a || b > 1000000000) {
            System.out.println("-1");
            return;
        }
        
        int lenA = intlen(a);
        int lenB = intlen(b);
        
        for(int i = lenA; i <= lenB; i++) {
            char[] cs = new char[i];
            generate(cs, 0, i);
        }
        
        System.out.println(count);
    }
}

def intlen(num):
    """计算数字的位数"""
    i = 0
    while num:
        num //= 10
        i += 1
    return i

def generate(cs, index, length, a, b):
    """递归生成数字"""
    if length >= 10:
        return 0
        
    if index == length:
        num = int(''.join(cs[:length]))
        if a <= num <= b:
            return 1
        return 0
    
    count = 0
    for digit in ['6', '8']:
        cs[index] = digit
        count += generate(cs, index + 1, length, a, b)
    return count

def main():
    # 输入并检查合法性
    a, b = map(int, input().split())
    if a < 1 or a > 1000000000:
        print("-1")
        return
        
    if b < a or b > 1000000000:
        print("-1")
        return
    
    # 计算位数范围
    len_a = intlen(a)
    len_b = intlen(b)
    
    # 统计所有可能的组合
    total = 0
    for length in range(len_a, len_b + 1):
        cs = ['0'] * length
        total += generate(cs, 0, length, a, b)
    
    print(total)

if __name__ == "__main__":
    main()