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