Toy problemsは役立たずか（Floydの問題・ネタバレ） http://blog.unfindable.net/archives/6103

floyd.cpp

//Are Toy Problems Useful?
//in Selected Papers on Computer Science by Knuth (p.172)

#include <iostream>
#include <sstream>
#include <iomanip>
#include <vector>
#include <set>
#include <unordered_map> // faster than map
#include <algorithm>
#include <ctime>
//#define SQRT sqrtl
//typedef long double real;
#define SQRT sqrt
typedef double real;
typedef unsigned int uint;
using namespace std;

const real DELTA = 1000.;
const uint N = 50;
const uint sizeA = 20;

int getKey(real x)
{
  return static_cast<int>((x - floor(x)) * 1e8);
}

real getSum(uint code, vector<real>* group, bool ignoreLast)
{
  real sum = 0;
  uint size = group->size();
  if (ignoreLast) --size;
  for (uint i = 0; i < size; ++i) if (code >> i & 1) sum += (*group)[i];
  return sum;
}

void printSolution(set<int>* solution)
{
  real sum = 0;
  stringstream ss;
  for (auto i : *solution) {
    sum += SQRT(i);
    ss << i << ",";
  }
  string str = ss.str();
  cout << setprecision(30) << sum << ": {" << str.substr(0, str.size()-1) << "}" << endl;
}

int main()
{
  auto start = clock();

  real dummy;
  real goal = 0;
  vector<int> all, a, b, c;
  vector<real> groupA, groupB, groupC; //平方根を計算しておく
  for (uint i = 1; i <= N; ++i) {
    all.push_back(i); //{1, 2, ..., 50}
    real t = SQRT(i);
    goal += t;
    if (t == floor(t)) {
      c.push_back(i); //{1, 4, 9, 16, 25, 36, 49}
      groupC.push_back(t);
    }
    else if (a.size() < sizeA) {
      a.push_back(i); //{2, 3, 5, 6, 7, 8, 10, 11, 12, 13,...
      groupA.push_back(t);
    }
    else {
      b.push_back(i);
      groupB.push_back(t);
    }
  }
  goal /= 2.;
  real fracPartOfGoal = modf(goal, &dummy);

  //aのsubset sumを計算し、記憶する
  unordered_multimap<int, pair<int, real>* > subsetSumOfA; //キー：概数、値：codeAと和のペア
  for (uint codeA = 0; codeA < (1u << groupA.size()); ++codeA) {
    real sumA = getSum(codeA, &groupA, false);
    int key = getKey(sumA);
    auto p = new pair<int, real>(codeA, sumA);
    subsetSumOfA.insert(pair<int, pair<int, real>* >(key, p));
  }

  //bのsubset sumを計算する
  real minDelta = DELTA;
  int codeAforMinDelta = 0, codeBforMinDelta = 0;
  real sumAforMinDelta = 0, sumBforMinDelta = 0;
//#pragma omp parallel
  for (uint codeB = 0; codeB < (1u << (groupB.size() - 1)); ++codeB) { //bの1つの要素は決めておいてよい
    real sumB = getSum(codeB, &groupB, true);
    real t = fabs(fracPartOfGoal - modf(sumB, &dummy)); //興味があるのは非常に近い場合だけだからこれでいい
    auto range = subsetSumOfA.equal_range(getKey(t));
    for (auto i = range.first; i != range.second; ++i) {
      int codeA = i->second->first;
      real sumA = i->second->second;
      real delta = fabs(fracPartOfGoal - modf(sumA, &dummy) - modf(sumB, &dummy));
      if (delta < minDelta) {
//#pragma omp critical
        {
          minDelta = delta;
          codeAforMinDelta = codeA;
          codeBforMinDelta = codeB;
          sumAforMinDelta = sumA;
          sumBforMinDelta = sumB;
        }
      }
    }
  }

  //整数部分を揃える
  real sum = sumAforMinDelta + sumBforMinDelta;
  minDelta = DELTA;
  int codeCforMinDelta = 0;
  for (uint codeC = 0; codeC < (1u << groupC.size()); ++codeC) {
    real delta = fabs(goal - sum - getSum(codeC, &groupC, false));
    if (delta < minDelta) {
      minDelta = delta;
      codeCforMinDelta = codeC;
    }
  }

  std::cout << setprecision(30) << goal << ": goal" << endl;
  auto solution1 = set<int>(); //ソート済みになる
  auto solution2 = set<int>();
  for (uint i = 0; i < groupA.size(); ++i) if (codeAforMinDelta >> i & 1) solution1.insert(a[i]);
  for (uint i = 0; i < groupB.size() - 1; ++i) if (codeBforMinDelta >> i & 1) solution1.insert(b[i]);
  for (uint i = 0; i < groupC.size(); ++i) if (codeCforMinDelta >> i & 1) solution1.insert(c[i]);
  set_difference(all.begin(), all.end(), solution1.begin(), solution1.end(), inserter(solution2, solution2.end()));
  printSolution(&solution1);
  printSolution(&solution2);

  clock_t finish = clock();
  real duration = real(finish - start) / CLOCKS_PER_SEC;
  cout << "Elapsed time: " << setprecision(3) << duration << " sec." << endl;
  return 0;
}