习题10-4 素数间隔 UVa1644

1.题目描述：点击打开链接

2.解题思路：根据题意可知最大的素数在int范围内，可以先算出1299709以内的所有素数，随后二分查找n附近的素数的位置即可。

3.代码：

#define _CRT_SECURE_NO_WARNINGS
#include<iostream>
#include<algorithm>
#include<string>
#include<sstream>
#include<set>
#include<vector>
#include<stack>
#include<map>
#include<queue>
#include<deque>
#include<cstdlib>
#include<cstdio>
#include<cstring>
#include<cmath>
#include<ctime>
#include<functional>
using namespace std;

#define N 1300000
int vis[N];
vector<int>primes;
void init()
{
	int m = sqrt(N + 0.5);
	for (int i = 2; i <= m;i++)
	if (!vis[i])
	for (int j = i*i; j < N; j += i)
		vis[j] = 1;
	for (int i = 2; i <= N;i++)
	if (!vis[i])
		primes.push_back(i);
}
int main()
{
	//freopen("test.txt", "r", stdin);
	int n;
	init();
	while (scanf("%d", &n) != EOF&&n)
	{
		if (!vis[n])cout << 0 << endl;
		else
		{
			int L = 0, R = 100000;
			while (L < R)
			{
				int m = L + (R - L) / 2;
				if (primes[m] > n)R = m;
				else L = m + 1;
			}
			cout << primes[L] - primes[L - 1] << endl;
		}
	}
	return 0;
}