HDU 4548.美素数 解题心得

#include<iostream>
#include<cmath>
#include<cstdio>
using namespace std;
const int N = 1000000 + 10;
int ans[N] = { 0 };
int vis[N];

bool check(int x)
{
    int total=0;
    while (1)
    {
        total += (x % 10);
        //cout << total << endl;
        x = x / 10;
        if (x == 0)
            break;

    }
    if (vis[total] == 0)
        return 1;
    else
        return 0;
}
int main()
{
    int t, kase = 1;
    cin >> t;

    vis[1] = 1;
    for (int i = 2; i <= N; i++)
    {
        if (vis[i] == 0)
        for (int j = 2; j*i <= N; j++)
        {
            vis[j*i] = 1;
        }
    }

    for (int i = 1; i <= N; i++)
    {
        if (vis[i] == 0&&check(i))
            ans[i] = ans[i - 1] + 1;
        else
            ans[i] = ans[i - 1];
    }
    while (t--)
    {
        int cnt = 0;
        int left,right;
        cin >> left >> right;
        printf("Case #%d: %d\n", kase++, ans[right]-ans[left-1]);
    
    }
    return 0;
}