Move Project Euler under puzzles.

puzzles/project_euler/p001_MultipleOf3and5.cpp

@@ -0,0 +1,39 @@
+/* Check cf5-opt.vim defs.
+VIM: let g:lcppflags="-std=c++11 -O2 -pthread"
+VIM: let g:wcppflags="/O2 /EHsc /DWIN32"
+*/
+#include <iostream>
+
+/*
+Multiples of 3 and 5
+Problem 1
+
+If we list all the natural numbers below 10 that are multiples of 3 or 5, we
+get 3, 5, 6 and 9. The sum of these multiples is 23.
+
+Find the sum of all the multiples of 3 or 5 below 1000.
+
+Solution:
+233168
+*/
+
+/*
+	Sum of all numbers below n multiple of m
+*/
+int sum(int n, int m )
+{
+	int c = n/m;
+	return m*c*(c+1)/2;
+}
+
+int main ( void )
+{
+	const int l = 999;
+	const int o1 = 3;
+	const int o2 = 5;
+	int s =	sum( l, o1 ) + sum( l, o2 ) - sum( l, o1*o2 );
+	std::cout << s << std::endl;
+
+	return 0;
+}
+