Submission #302705

#	Time	Username	Problem	Language	Result	Execution time	Memory
302705		user202729	Towns (IOI15_towns)	C++17	25 / 100	26 ms	896 KiB

towns

// moreflags=grader.cpp
// really, who includes .c files in a .cpp one?

// ...
// definitely not an excuse for not learning classical algorithms properly.

// !! why is the value in the towns.out file in the zip 17?

#include "towns.h"
#include<algorithm>
#include<climits>
#include<map>
#include<vector>
#if not LOCAL
#define NDEBUG
#endif
#include<cassert>

int hubDistance(int number, int sub) {
	int const A=0; // * must be 0, relied upon by some code below
	std::vector<int> distanceA(number);
	for(int other=1; other<number; ++other)
		distanceA[other]=getDistance(A, other);

	int const B=int(std::max_element(begin(distanceA), end(distanceA))-distanceA.begin());
	std::vector<int> distanceB(number);
	distanceB[A]=distanceA[B];
	for(int other=1; other<number; ++other) if(other!=B)
		distanceB[other]=getDistance(B, other);
	auto const diameter=*std::max_element(begin(distanceB), end(distanceB));

	// distance from B -> [(distance from path AB, node index)...]
	std::map<int, std::vector<std::pair<int, int>>> groups;
	for(int node=0; node<number; ++node){
		auto const first=distanceA[node], sec=distanceB[node];
		assert((first+sec+distanceA[B])%2==0);
		groups[(sec+distanceA[B]-first)>>1].push_back({(first+sec-distanceA[B])>>1, node});
	}
	assert((groups.at(0)==std::vector<std::pair<int, int>>{{0, B}}));
	assert((groups.at(distanceA[B])==std::vector<std::pair<int, int>>{{0, A}}));
	// ^ technically have side effects (memory allocation and deallocation)

	int radius=INT_MAX, hubDistanceB; bool LRbalanced=false;
	{
		int sumLeftGroupSize=0; // exclude current group. Only count leaves
		int lastGroupSize, lastr, lastDistanceB;
		for(auto const& [distanceB, group]: groups){
			auto const r=std::max(distanceB, diameter-distanceB); // only correct for nodes on some diameter

			if(distanceB*2>=diameter){
				// node and lastNode are on some diameter
				// but the next node might not be
				// compare node and lastNode. One of them is the hub

				radius=std::min(lastr, r);
				bool const lastLRBalanced=
					std::max(sumLeftGroupSize-lastGroupSize, number-sumLeftGroupSize)
					<=number/2;
				bool const curLRBalanced=
					std::max(sumLeftGroupSize, number-sumLeftGroupSize-(int)group.size())
					<=number/2;
				assert(not (lastLRBalanced and curLRBalanced));

				if(radius==lastr){
					assert(not LRbalanced);
					hubDistanceB=lastDistanceB; LRbalanced=lastLRBalanced;
				}
				if(radius==r and curLRBalanced>=LRbalanced){ // if there are two, use the LRbalanced one
					hubDistanceB=distanceB; LRbalanced=curLRBalanced;
				}

				break;
			}
			lastGroupSize=(int)group.size(); lastr=r; lastDistanceB=distanceB;
			sumLeftGroupSize+=(int)group.size();
		}
	}

	assert(radius!=INT_MAX);
	if(sub<3){
		return radius;
	}
}

Compilation message (stderr)

towns.cpp: In function 'int hubDistance(int, int)':
towns.cpp:47:36: warning: declaration of 'distanceB' shadows a previous local [-Wshadow]
   47 |   for(auto const& [distanceB, group]: groups){
      |                                    ^
towns.cpp:26:19: note: shadowed declaration is here
   26 |  std::vector<int> distanceB(number);
      |                   ^~~~~~~~~
towns.cpp:43:22: warning: variable 'hubDistanceB' set but not used [-Wunused-but-set-variable]
   43 |  int radius=INT_MAX, hubDistanceB; bool LRbalanced=false;
      |                      ^~~~~~~~~~~~
towns.cpp:21:35: warning: control reaches end of non-void function [-Wreturn-type]
   21 |  std::vector<int> distanceA(number);
      |                                   ^
towns.cpp:46:22: warning: 'lastr' may be used uninitialized in this function [-Wmaybe-uninitialized]
   46 |   int lastGroupSize, lastr, lastDistanceB;
      |                      ^~~~~

• Subtask 1: 13 / 13
• Subtask 2: 12 / 12
• Subtask 3: 0 / 13
• Subtask 4: 0 / 10
• Subtask 5: 0 / 13
• Subtask 6: 0 / 39