Knapsack solution with Backtraking in c++

Question

Im having troubles trying to resolve the Knapsack problem using backtraking.

For example, for the following values, the Knapsack function will return 14 as the solution, but the correct result should be 7.

int n = 3, weights[] = {2, 3, 1}, values[] = {6, 15, 7};

int solution = 0, max = 2;


void Knapsack (int i, int max, int value, int *solution)
{
  int k;

  for (k = i; k < n; k++) {
    if (weights[k] <= max) {
      Knapsack (k, max - weights[k], value + values[k], solution);

      if (value+ values[k] > *solution) 
         *solution= value + values[k];
    }
  }
}

What is the problem here?

Answer 1

#include<iostream>
#include<vector>

using namespace std;

int weights[] = {2, 3, 1}, values[] = {6, 15, 7};

int solution = 0, n = 3;

std::vector<int> vsol;
std::vector<int> temp;

bool issol;


void Knapsack (int i, int max, int value)
{
  for (int k = i; k < n; k++) {
    if ( max > 0)
    {
        if (weights[k] <= max)
        {
          temp.push_back(k);
          if (value+ values[k] >= solution)
          {
            solution = value + values[k];
            issol = true;
          }
        }
        if ( (k+1) < n)
        {
          Knapsack (k+1, max - weights[k], value + values[k]);
        }
        else
        {
          if (issol == true)
          {
            if (! vsol.empty()) vsol.clear();
            std::move(temp.begin(), temp.end(), std::back_inserter(vsol));
            temp.clear();
            issol = false;
          } else temp.clear();
          return;
        }
    }
    else
    {
        if (issol == true)
        {
            if (! vsol.empty()) vsol.clear();
            std::move(temp.begin(), temp.end(), std::back_inserter(vsol));
            temp.clear();
            issol = false;
        } else temp.clear();
        return;
    }
  }
}

int main()
{
    Knapsack(0, 2, 0);
    cout << "solution: " << solution << endl;
    for(vector<int>::iterator it = vsol.begin(); it != vsol.end(); it++)
        cout << *it << " ";
    return 0;
}

You need to increase k by 1 when you call the Knapsack function again to move the index forward.

Added code to print out index locations of the solution. If more than one solution exists (i.e. same total), will only print out the locations for the last solution.