Interviews are considered puzzles.

puzzles/interviews/FacebookInterviewTest.cpp

@@ -0,0 +1,60 @@
+/* Check cf5-opt.vim defs.
+VIM: let g:lcppflags="-std=c++11 -O2 -pthread"
+VIM: let g:wcppflags="/O2 /EHsc /DWIN32"
+VIM-: let g:cppflags=g:Iboost.g:Itbb
+VIM-: let g:ldflags=g:Lboost.g:Ltbb.g:tbbmalloc.g:tbbmproxy
+VIM-: let g:ldlibpath=g:Bboost.g:Btbb
+VIM: let g:argv=""
+*/
+#include <iostream>
+#include <vector>
+#include <stdexcept>
+
+int main ( void )
+{try{
+
+	//
+	//	Read length.
+	//
+	long long n;
+	std::cin >> n;
+	if ( n < 4 )
+		throw std::runtime_error( "Input length is not enough." );
+	//
+	//	Read seq.
+	//
+	typedef std::vector<int> arr_type;
+	arr_type arr(n);
+	for ( int& i: arr )
+		std::cin >> i;
+	//
+	//	Solve it.
+	//
+	long long s = 0;
+	for ( long long i: arr )
+		s += i;
+	long long d = arr[1]-arr[0];
+	if ( d != arr[2]-arr[1] && d != arr[3]-arr[2])
+		d = arr[2]-arr[1];
+	long long idx = (n+1)*n/2-(s-n*arr[0])/d;
+	//
+	//
+	//
+	long long missing = arr[0]+idx*d;
+	std::cout << missing;
+
+	return 0;
+}
+catch ( const std::exception& e )
+{
+	std::cerr << std::endl
+			<< "std::exception(\"" << e.what() << "\")." << std::endl;
+	return 2;
+}
+catch ( ... )
+{
+	std::cerr  << std::endl
+			<< "unknown exception." << std::endl;
+	return 1;
+}}
+