E. Jzzhu and Apples

http://codeforces.com/contest/450/problem/E

编号大的优先算入结果，质因数大的优先算入结果

 1 public class Main {
 2     static final int maxn = (int) 1e5 + 100;
 3 
 4     static int n;
 5     static int[] isPrime = new int[maxn];
 6 
 7     public static void main(String[] args) {
 8         IO io = new IO();
 9         n = io.nextInt();
10         primeTable(n);
11 
12         int[] g = new int[maxn];
13         int[] vis = new int[maxn];
14         ArrayList<int[]> ans = new ArrayList<>();
15 
16         //质因数大的优先算入结果
17         for (int i = n/2; i>=2; i--) {
18             if (isPrime[i] == 0) continue;
19             g[0]=0;
20             for (int j = i; j <= n; j += i) if (vis[j] == 0) g[++g[0]] = j;
21             //编号大的优先算入结果，保留最小的数字，不能是第一个，因为第一个是素数，所以保留第二个
22             if (g[0]%2==1){int t=g[2];g[2]=g[g[0]];g[g[0]]=t;}
23             for (int j = 1; j + 1 <= g[0]; j += 2) {
24                 ans.add(new int[]{g[j], g[j + 1]});
25                 vis[g[j]] = vis[g[j + 1]] = 1;
26             }
27         }
28         io.println(ans.size());
29         for (int[]i:ans)io.println(i[0]+" "+i[1]);
30     }
31 
32     static void primeTable(int n) {
33         Arrays.fill(isPrime, 1);
34         for (int i = 2; i * i <= n; i++) {
35             if (isPrime[i] == 0) continue;
36             for (int j = i * i; j <= n; j += i) isPrime[j] = 0;
37         }
38     }
39 
40    
41 }

标签：算入,int,Jzzhu,static,maxn,Apples,io,new
来源： https://www.cnblogs.com/towerbird/p/11238518.html