Move Project Euler under puzzles.

puzzles/project_euler/p007_10001st_prime.cpp

@@ -0,0 +1,45 @@
+/* Check cf5-opt.vim defs.
+VIM: let g:lcppflags="-std=c++11 -O2 -pthread"
+VIM: let g:wcppflags="/O2 /EHsc /DWIN32"
+*/
+#include <iostream>
+#include <vector>
+
+/*
+10001st prime
+Problem 7
+
+By listing the first six prime numbers: 2, 3, 5, 7, 11, and 13, we can see that the 6th prime is 13.
+
+What is the 10 001st prime number?
+
+Solution:
+104743
+*/
+
+typedef std::vector<long long> pt;
+pt p;
+
+bool isPrime( long long n )
+{
+	long h = (long long)sqrt(double(n));
+	for ( pt::iterator i = p.begin(), ie = p.end(); 
+			i!=ie && *i<=h;
+			++i )
+		if ( !(n%*i) )
+			return false;
+	return true;
+}
+
+int main ( void )
+{
+	const long long n = 10001;
+
+	for ( long long i = 2; p.size() < n; ++ i )
+		if ( isPrime( i ) )
+		   p.push_back( i );
+	std::cout << p.back() << std::endl;
+
+	return 0;
+}
+