http://www.spoj.com/problems/SUMFOUR/

sumfour.cpp

#include <fstream>
#include <iostream>
#include <unordered_map>
#include <vector>

int main() {
  std::ifstream datafile("data.txt");
  int64_t numlists;
  int64_t a, b, c, d;
  std::vector<int64_t> A, B, C, D;

  datafile >> numlists;
  while (datafile >> a >> b >> c >> d) {
    A.push_back(a);
    B.push_back(b);
    C.push_back(c);
    D.push_back(d);
  }

  std::unordered_map<int64_t, int64_t> AB, CD;

  // Compute all pairs of a + b
  for (auto a : A) {
    for (auto b : B) {
      try {
        AB.at(a + b) += 1;
      } catch (const std::out_of_range& oor) {
        AB[a + b] = 1;
      }
    }
  }
  
  // Compute all pairs of c + d
  for (auto c : C) {
    for (auto d : D) {
      try {
        CD.at(c + d) += 1;
      } catch (const std::out_of_range& oor) {
        CD[c + d] = 1;
      }
    }
  }
  
  // Do there exist (c + d) == -(a + b)? How many?
  int result = 0;
  for (auto i : AB) {
    try {
      // Accounts for possibilities were distinct are distinct combinations of
      // a, b result in equal sums
      result += i.second * CD.at(-(i.first));
    } catch (const std::out_of_range& oor) {}
  }

  std::cout << "result: " << result << std::endl;
  return 0;
}

/*
data.txt

6
-45 22 42 -16
-41 -27 56 30
-36 53 -37 77
-36 30 -75 -46
26 -38 -10 62
-32 -54 -6 45
*/