[51NOD1181]质数中的质数（质数筛法）（欧拉筛）

Posted 2020-07-25

题目链接：http://www.51nod.com/onlineJudge/questionCode.html#!problemId=1181

思路：欧拉筛出所有素数和一个数的判定，找到大于n的最小质数序号p，并且判断p是不是质数，输出这个数。

 1     /*
 2 ━━━━━┒ギリギリ♂ eye！
 3 ┓┏┓┏┓┃キリキリ♂ mind！
 4 ┛┗┛┗┛┃＼○／
 5 ┓┏┓┏┓┃ /
 6 ┛┗┛┗┛┃ノ)
 7 ┓┏┓┏┓┃
 8 ┛┗┛┗┛┃
 9 ┓┏┓┏┓┃
10 ┛┗┛┗┛┃
11 ┓┏┓┏┓┃
12 ┛┗┛┗┛┃
13 ┓┏┓┏┓┃
14 ┃┃┃┃┃┃
15 ┻┻┻┻┻┻
16 */
17 #include <algorithm>
18 #include <iostream>
19 #include <iomanip>
20 #include <cstring>
21 #include <climits>
22 #include <complex>
23 #include <fstream>
24 #include <cassert>
25 #include <cstdio>
26 #include <bitset>
27 #include <vector>
28 #include <deque>
29 #include <queue>
30 #include <stack>
31 #include <ctime>
32 #include <set>
33 #include <map>
34 #include <cmath>
35 using namespace std;
36 #define fr first
37 #define sc second
38 #define cl clear
39 #define BUG puts("here!!!")
40 #define W(a) while(a--)
41 #define pb(a) push_back(a)
42 #define Rint(a) scanf("%d", &a)
43 #define Rll(a) scanf("%lld", &a)
44 #define Rs(a) scanf("%s", a)
45 #define Cin(a) cin >> a
46 #define FRead() freopen("in", "r", stdin)
47 #define FWrite() freopen("out", "w", stdout)
48 #define Rep(i, len) for(int i = 0; i < (len); i++)
49 #define For(i, a, len) for(int i = (a); i < (len); i++)
50 #define Cls(a) memset((a), 0, sizeof(a))
51 #define Clr(a, x) memset((a), (x), sizeof(a))
52 #define Full(a) memset((a), 0x7f7f7f, sizeof(a))
53 #define lrt rt << 1
54 #define rrt rt << 1 | 1
55 #define pi 3.14159265359
56 #define RT return
57 #define lowbit(x) x & (-x)
58 #define onenum(x) __builtin_popcount(x)
59 typedef long long LL;
60 typedef long double LD;
61 typedef unsigned long long ULL;
62 typedef pair<int, int> pii;
63 typedef pair<string, int> psi;
64 typedef pair<LL, LL> pll;
65 typedef map<string, int> msi;
66 typedef vector<int> vi;
67 typedef vector<LL> vl;
68 typedef vector<vl> vvl;
69 typedef vector<bool> vb;
70 
71 const int maxn = 1000010;
72 int n, pcnt;
73 bool isprime[maxn];
74 int prime[maxn];
75 
76 int main() {
77     // FRead();
78     Cls(prime); Clr(isprime, true); pcnt = 0;
79     For(i, 2, maxn) {
80         if(isprime[i]) prime[++pcnt] = i;
81         For(j, 1, pcnt+1) {
82             if(i * prime[j] > maxn) break;
83             isprime[i*prime[j]] = 0;
84             if(i % prime[j] == 0) break;
85         }
86     }
87     while(~Rint(n)) {
88         int p = 1;
89         while(prime[p] < n) p++;
90         while(1) {
91             if(isprime[p]) break;
92             p++;
93         }
94         printf("%d\n", prime[p]);
95     }
96     RT 0;
97 }