luckytickets.cpp

#include <array>
#include <vector>
#include <iostream>

auto main()->int {
  // use a sparse array to hold sums
  std::array<std::vector<std::array<unsigned int, 3>>, 9 * 9 * 9> sums;

  size_t maxsize = 0;
  // first populate the sparse array
  std::array<unsigned int, 3> triplet;
  for(triplet[0] = 0; triplet[0] != 10; ++triplet[0]) {
    for(triplet[1] = 0; triplet[1] != 10; ++triplet[1]) {
      for(triplet[2] = 0; triplet[2] != 10; ++triplet[2]) {
        unsigned int const sum = triplet[0] + triplet[1] + triplet[2];
        // add this triplet to the cache for this sum
        sums[sum].emplace_back(triplet);
        maxsize = std::max(maxsize, sums[sum].size());
      }
    }
  }
  std::cout << "Max triplets with the same sum: " << maxsize << std::endl;

  // then iterate through the sums and list all triplet combinations that match
  unsigned int counter = 0;
  for(auto const &sum : sums) {
    for(auto const &left : sum) {
      for(auto const &right : sum) {
        std::cout << left[0] << left[1] << left[2] << right[0] << right[1] << right[2] << std::endl;
        ++counter;
      }
    }
  }

  std::cout << "Total: " << counter << std::endl;

  return EXIT_SUCCESS;
}