UVa 1644 Prime Gap (水题，暴力)

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;
}