10.27 Gift

题目：

解题思路：

01背包
转移方程：
$(f[i+1][j] + f[i+1][j-a[i]] * (j>=a[i]))$(f[i+1][j]+f[i+1][j−a[i]]∗(j>=a[i]))

Accepted code:

#include<cstdio>
#include<algorithm>
#define YMW int(1e7+7)

using namespace std;

const int N = 2005;
int n, m, a[N], f[N][N];

int main() {
	scanf("%d %d", &n, &m); int sum = 0;
	for (int i = 1; i <= n; i++)
		scanf("%d", &a[i]), sum += a[i];
	if (sum <= m) return !putchar('1');
	sort(a + 1, a + n + 1);
	f[n+1][0] = 1;
	for (int i = n; i >= 1; i--)
		for (int j = 0; j <= m; j++)
			f[i][j] = (f[i+1][j] + f[i+1][j-a[i]] * (j>=a[i])) % YMW;
	int ans = 0, t = 0;
	for (int i = 1; i <= n; i++) {
		for (int j = 0; j < a[i]; j++)
			if (m-t-j >= 0)
				(ans += f[i+1][m-t-j]) %= YMW;
		t += a[i];
	}
	printf("%d", ans);
}