线性筛&一般筛

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一般筛，每个数被筛次数会出现>1，复杂度近似于O(NloglogN)：

//一般筛 var vis:array[0..1000005]of boolean;     i,j,n:longint; begin  assign(input,'xxs.in');reset(input);  assign(output,'xxs.out');rewrite(output);  fillchar(vis,sizeof(vis),1);  readln(n);  for i:=2 to trunc(sqrt(n)) do   if vis[i] then     for j:=2 to n div i do vis[i*j]:=false;  for i:=2 to n do    if vis[i] then writeln(i);  close(input);close(output); end.

线性筛，关键在于if (i%prime[j]==0) break，就避免了一个数被重复筛，复杂度O(N)：

//线性筛 #include<cstdio>#include<cstring>int prime[1000005];int n,cnt;bool vis[1000005];using namespace std;int main(){freopen("xxs.in","r",stdin);freopen("xxs1.out","w",stdout);int n,cnt;scanf("%d",&n);cnt=0;memset(vis,0,sizeof(vis));for (int i=2;i<=n;i++){if (!vis[i])prime[++cnt]=i;for (int j=1;j<=cnt&&prime[j]*i<=n;j++){vis[prime[j]*i]=1;if (i%prime[j]==0) break;}}for (int i=1;i<=cnt;i++) printf("%d\n",prime[i]);return 0;}

【写的有漏洞的，欢迎路过大神吐槽】

2016/12/22 17:40:40

Ending.

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