Codeforces 449C Jzzhu and Apples 贪心 (看题解）

Jzzhu and Apples

从大的质因子开始贪心， 如果有偶数个则直接组合， 如果是奇数个留下那个质数的两倍， 其余两两组合。

#include<bits/stdc++.h>
#define LL long long
#define fi first
#define se second
#define mk make_pair
#define PLL pair<LL, LL>
#define PLI pair<LL, int>
#define PII pair<int, int>
#define SZ(x) ((int)x.size())
#define ull unsigned long long

using namespace std;

const int N = 1e5 + 7;
const int inf = 0x3f3f3f3f;
const LL INF = 0x3f3f3f3f3f3f3f3f;
const int mod = 1e9 + 7;
const double eps = 1e-8;
const double PI = acos(-1);

int n;
bool vis[N];
bool prime[N];
queue<int> que;
vector<PII> ans;

int main() {
    memset(prime, true, sizeof(prime));
    prime[0] = prime[1] = false;
    for(int i = 2; i < N; i++) {
        if(!prime[i]) continue;
        for(int j = i * 2; j < N; j += i)
            prime[j] = false;
    }
    scanf("%d", &n);
    for(int i = n; i >= 2; i--) {
        if(!prime[i]) continue;
        for(int j = 3 * i; j <= n; j += i)
            if(!vis[j]) que.push(j);
        que.push(i);
        if(2 * i <= n && !vis[2 * i]) que.push(2 * i);
        while(SZ(que) >= 2) {
            int x = que.front(); que.pop();
            int y = que.front(); que.pop();
            vis[x] = vis[y] = true;
            ans.push_back(mk(x, y));
        }
        while(SZ(que)) que.pop();
    }
    printf("%d
", SZ(ans));
    for(auto& t : ans) printf("%d %d
", t.fi, t.se);
    puts("");
    return 0;
}

/*
*/