USACO 2015 US Open, Bronze Problem 2. Bessie Gets Even

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USACO2015OPEN-B2

答案

(Analysis by Nick Wu)

In a pure brute-force solution, we would try every possible combination of assignments of variables to values. There are 7 variables, with at most 20 values per variable, for a total of 207207 combinations. This is over one billion combinations to check, which is too many to check.

One approach you can try is to count the number of ways you can force the expression to be odd. When checking if a combination is odd, you can immediately note a couple things - for example, M must be odd. Also, if you recursively assign values to variables and you see that one of the three terms in the product is even, you can stop all combinations for variables that you haven't yet inspected.

There is a much faster approach though that removes the dependency on checking different combinations. Since you want to check if the product is even or odd, the important thing to know for each variable is how many even values that variable can take on, and how many odd values that variable can take on. Once you've done that, you can assign to each variable a parity and see if with those parities, the product is even. If so, you can count how many combinations there are with those parities, and then sum the parities.

With this approach, there are only 27=12827=128 combinations of parities to check, which is guaranteed to work quickly enough.

Here is Mark Gordon's code:

#include <iostream>
#include <cstdio>

using namespace std;

int num[256][2];

bool is_even(int x) {
  return x % 2 == 0;
}

int main() {
  freopen("geteven.in", "r", stdin);
  freopen("geteven.out", "w", stdout);

  int N;
  cin >> N;

  for (int i = 0; i < N; i++) {
    char letter;
    int val;
    cin >> letter >> val;

    if (is_even(val)) {
      num[letter][0]++;
    } else {
      num[letter][1]++;
    }
  }

  int result = 0;

  /* Try every possible way that the variables could be even or odd. */
  for(int B = 0; B < 2; B++)
  for(int E = 0; E < 2; E++)
  for(int S = 0; S < 2; S++)
  for(int I = 0; I < 2; I++)
  for(int G = 0; G < 2; G++)
  for(int O = 0; O < 2; O++)
  for(int M = 0; M < 2; M++) {
    if (is_even((B + E + S + S + I + E) * (G + O + E + S) * (M + O + O))) {
      /* If the expression is even then add the number of variable assignments
       * that have the variables odd/even.
       */
      result += num['B'][B] * num['E'][E] * num['S'][S] * num['I'][I] *
                num['G'][G] * num['O'][O] * num['M'][M];
    }
  }
  cout << result << endl;

  return 0;
}

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