题意:给定一个数 n,求它后一个素数和前一个素数差。
析:先打表,再二分查找。
代码如下:
#pragma comment(linker, "/STACK:1024000000,1024000000") #include <cstdio> #include <string> #include <cstdlib> #include <cmath> #include <iostream> #include <cstring> #include <set> #include <queue> #include <algorithm> #include <vector> #include <map> #include <cctype> #include <cmath> #include <stack> #define print(a) printf("%d\n", (a)) #define freopenr freopen("in.txt", "r", stdin) #define freopenw freopen("out.txt", "w", stdout) using namespace std; typedef long long LL; typedef pair<int, int> P; const int INF = 0x3f3f3f3f; const double inf = 0x3f3f3f3f3f3f; const LL LNF = 0x3f3f3f3f3f3f; const double PI = acos(-1.0); const double eps = 1e-8; const int maxn = 1300000 + 5; const int mod = 1e9 + 7; const int dr[] = {-1, 0, 1, 0}; const int dc[] = {0, 1, 0, -1}; const char *Hex[] = {"0000", "0001", "0010", "0011", "0100", "0101", "0110", "0111", "1000", "1001", "1010", "1011", "1100", "1101", "1110", "1111"}; int n, m; const int mon[] = {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}; const int monn[] = {0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}; inline int Min(int a, int b){ return a < b ? a : b; } inline int Max(int a, int b){ return a > b ? a : b; } inline LL Min(LL a, LL b){ return a < b ? a : b; } inline LL Max(LL a, LL b){ return a > b ? a : b; } inline bool is_in(int r, int c){ return r >= 0 && r < n && c >= 0 && c < m; } int a[maxn]; vector<int> prime; int main(){ memset(a, 0, sizeof(a)); m = (int)sqrt(maxn + 0.5); for(int i = 2; i <= m; i++) if(!a[i]) for(int j = i * i; j < maxn; j += i) a[j] = 1; for(int i = 2; i < maxn; ++i) if(!a[i]) prime.push_back(i); while(cin >> n && n){ if(!a[n]) printf("0\n"); else{ int pos = lower_bound(prime.begin(), prime.end(), n) - prime.begin(); printf("%d\n", prime[pos]-prime[pos-1]); } } return 0; }
时间: 2024-10-29 10:45:45