One general tip is to always do quick, back-of-the-envelope calculations about whether you can calculate something or not. For example, we might naturally wonder whether we can just simulate all possible scenarios Farmer John can actually create. To figure out whether we can, we need to know how many different scenarios are possible.

On Monday, FJ can choose from 10 different buckets. On Tuesday, he will be able to choose from 11 (no matter which bucket he brings); on Wednesday, Thursday, and Friday, he will also have 11 choices. Thus, a rough upper bound for the number of different things Farmer John can do is $10 * 11^4 = 146410$ operations, which means we can just simulate them. (A good rule of thumb is that if the number is under 20,000,000, it will probably run in time. This is *far* below that number!)

To do this, we can keep two arrays "B1" and "B2" representing the buckets in barns B1 and B2, respectively. We first call a function called "tuesday" which tries all possible values in B1, then passes the new values of B1 and B2 to a function called "wednesday". We repeat with functions "wednesday", "thursday", and "friday", keeping track of the milk at the first barn.

Since the possible ending values of milk are all between $0$ and $2,000$ (a very conservative estimate), we can keep an array of size $2,000$ and flip its values from false to true depending on whether we can. At the end, we count all the "true" values of the array to get our final answer.

Here is Brian's solution, written in a way that deliberately tries to match the problem structure as much as possible:

#include <iostream> #include <fstream> #include <vector> using namespace std; int possible_answers[2000]; void friday(int b1milk, vector<int> B1, vector<int> B2) { for (int i=0; i<B2.size(); i++) { int x = B2[i]; possible_answers[b1milk + x] = 1; // record this value as possible } } void thursday(int b1milk, vector<int> B1, vector<int> B2) { for (int i=0; i<B1.size(); i++) { int x = B1[i]; vector<int> new_B2 = B2; new_B2.push_back(x); vector<int> new_B1 = B1; new_B1.erase(new_B1.begin() + i); friday(b1milk - x, new_B1, new_B2); } } void wednesday(int b1milk, vector<int> B1, vector<int> B2) { for (int i=0; i<B2.size(); i++) { int x = B2[i]; vector<int> new_B1 = B1; new_B1.push_back(x); vector<int> new_B2 = B2; new_B2.erase(new_B2.begin() + i); thursday(b1milk + x, new_B1, new_B2); } } void tuesday(int b1milk, vector<int> B1, vector<int> B2) { for (int i=0; i<B1.size(); i++) { int x = B1[i]; vector<int> new_B2 = B2; new_B2.push_back(x); vector<int> new_B1 = B1; new_B1.erase(new_B1.begin() + i); wednesday(b1milk - x, new_B1, new_B2); } } int main(void) { int n, answer = 0; vector<int> B1, B2; ifstream fin ("backforth.in"); for (int i=0; i<10; i++) { fin >> n; B1.push_back(n); } for (int i=0; i<10; i++) { fin >> n; B2.push_back(n); } tuesday(1000, B1, B2); ofstream fout ("backforth.out"); for (int i=0; i<2000; i++) answer += possible_answers[i]; fout << answer << "\n"; return 0; }

For brownie points, you can even try combining the four functions into one function, and recursively calling that function. Here is a fancier solution by Dhruv which does this:

#include <iostream> #include <algorithm> #include <vector> #include <set> using namespace std; multiset<int> S[2]; // S[0] is barn 1, S[1] is barn 2 int pos[2001]; int numOutcomes; void dfs(int day,int amount) { // 'amount' represents the amount of milk in barn 1. // We know the amount of milk in barn 2 is 2000 - amount. if(day == 6) { numOutcomes += 1 - pos[amount]; pos[amount] = 1; return; } vector<int> vals; int p = (day%2); // 0 if 'day' is even, 1 otherwise. // This controls which element of S we use. multiset<int>::iterator it = S[p].begin(); while(it != S[p].end()) { vals.push_back(*it); it++; } for(int i=0;i<vals.size();i++) { S[p].erase(S[p].find(vals[i])); S[1-p].insert(vals[i]); if (p == 0) { dfs(day+1, amount - vals[i]); } else { dfs(day+1, amount + vals[i]); } S[1-p].erase(S[1-p].find(vals[i])); S[p].insert(vals[i]); } } int main() { int val; for(int p=0;p<2;p++) for(int i=0;i<10;i++) { cin >> val; S[p].insert(val); } dfs(2, 1000); cout << numOutcomes << '\n'; }