USACO2.2.2 Subset Sums (subset)

01背包水题。

如果集合中数字之和是奇数则直接输出0，否则就按背包的思路做。统计正好花费sum/2时的方案数。

/*ID:shijiey1PROG:subsetLANG:C++*/#include <cstdio>#include <cstring>#include <algorithm>using namespace std;int n, sum = 0;long long dp[50][5000];int main() {freopen("subset.in", "r", stdin);freopen("subset.out", "w", stdout);scanf("%d", &n);for (int i = 1; i <= n; i++) sum += i;if (sum % 2) {printf("0\n");return 0;}memset(dp, 0, sizeof(dp));dp[1][1] = 1;dp[1][0] = 1;for (int i = 2; i <= n; i++) {for (int j = 0; j <= sum; j++) {if (j >= i && dp[i - 1][j - i] != 0)dp[i][j] += dp[i - 1][j - i];if (dp[i - 1][j] != 0)dp[i][j] += dp[i - 1][j];}}printf("%lld\n", dp[n][sum / 2] / 2);return 0;}