蒙特卡洛方法

/*==========================================================================
# Last modified: 2016-03-14 11:46
# Filename: pi.cpp
# Description: 
==========================================================================*/
#define me AcrossTheSky 
#include <cstdio> 
#include <cmath> 
#include <ctime> 
#include <string> 
#include <cstring> 
#include <cstdlib> 
#include <iostream> 
#include <algorithm> 
  
#include <set> 
#include <map> 
#include <stack> 
#include <queue> 
#include <vector> 
 
#define lowbit(x) (x)&(-x) 
#define FOR(i,a,b) for((i)=(a);(i)<=(b);(i)++) 
#define FORP(i,a,b) for(int i=(a);i<=(b);i++) 
#define FORM(i,a,b) for(int i=(a);i>=(b);i--) 
#define ls(a,b) (((a)+(b)) << 1) 
#define rs(a,b) (((a)+(b)) >> 1) 
#define getlc(a) ch[(a)][0] 
#define getrc(a) ch[(a)][1] 
 
#define maxn 100000 
#define maxm 100000 
#define pi 3.1415926535898 
#define _e 2.718281828459 
#define INF 1070000000 
using namespace std; 
typedef long long ll; 
typedef unsigned long long ull; 
 
/*==================split line==================*/ 
 
int main(){ 
	srand(time(0));
	int cnt=0; 
	ll n=1000000000;
	FORP(i,1,n) {
		double x=rand()%1000,y=rand()%1000;
		x/=1000.0,y/=1000.0;
		if (sqrt(x*x+y*y)<=1) cnt++;
	}
	printf("%lf",(double)cnt/n*4);
}