competitive-programming/solved-lightOj/1259.cpp at master · the-redback/competitive-programming · GitHub

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/**
 *                                   ____
 *       ____ ___  ____ ________  __/ __/
 *      / __ `__ \/ __ `/ ___/ / / / /_
 *     / / / / / / /_/ / /  / /_/ / __/
 *    /_/ /_/ /_/\__,_/_/   \__,_/_/
 *
 *    @link : https://the-redback.com
 */

#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef unsigned long long llu;

#define ft        first
#define sd        second
#define mp        make_pair
#define pb(x)     push_back(x)
#define all(x)    x.begin(), x.end()
#define allr(x)   x.rbegin(), x.rend()
#define mem(a, b) memset(a, b, sizeof(a))
#define inf       1e9
#define eps       1e-9
#define mod       1000000007

#define read(a) scanf("%lld", &a)

#define NN 10000000
bool p[NN + 7];  // Hashing
vector<ll> pr;   // storing prime

void sieve(ll n) {
    ll i, j, k, l;
    p[1] = 1;
    pr.push_back(2);
    for (i = 4; i <= n; i += 2) p[i] = 1;
    for (i = 3; i <= n; i += 2) {
        if (p[i] == 0) {
            pr.push_back(i);
            for (j = i * i; j <= n; j += 2 * i) p[j] = 1;
        }
    }
}

int main() {
    sieve(NN);
    ll t = 1, tc;
    read(tc);
    ll i, j, k, l, m, n;
    while (tc--) {
        read(n);
        ll mid = n / 2;
        vector<ll>::iterator ind;
        ind = lower_bound(pr.begin(), pr.end(), mid);
        ll in = ind - pr.begin();
        ll low = in, high = in;
        ll ans = 0;
        while (low >= 0 && high < pr.size() && pr[high] < n) {
            // cout<<pr[low]<<" "<<pr[high]<<" "<<pr[low]+pr[high]<<"\n";
            if (pr[low] + pr[high] == n) {
                ans++;
                low--;
                high++;
            }
            if (pr[low] + pr[high] > n) low--;
            if (pr[low] + pr[high] < n) high++;
        }
        printf("Case %lld: %lld\n", t++, ans);
    }
    return 0;
}