为什么最后一个会RE呢

用的是埃氏筛。

#include<bits/stdc++.h>

using namespace std;

const int MAX = 9999999;//偶数位必然不是
int prime[MAX + 10];

void ai(){
    for (int i = 2; i <= MAX; i++) {
        prime[i] = 1;
    }
    for (int i = 2; i <= MAX; i++) {
        if (prime[i]) {
            for (int j = i * 2; j <= MAX; j += i){
                prime[j] = 0;
            }   
        }
    }
}
bool check (int x) {
    int a[20];
    int flag = 0;
    while (x) {
        a[flag] = x % 10;
        x /= 10;
        flag ++;
    }
    for (int i = 0;i < flag /2; i++){
        if (a[i] != a[flag - 1 - i])return 0;
    }
    return 1;
}
int main() {
    ai(); 
    int a, b;
    cin >> a >> b;
    for (int i = a; i <= b; i++) {
        if (check(i) && prime[i])cout << i << endl;
    }
    return 0;
}