✅ probability, mod

submissions/atCoder/abc323_e.cpp

@@ -0,0 +1,67 @@
+#include <iostream>
+
+const int mod(998244353);
+const int maxn(1'000);
+const int maxx(1'0'000);
+
+long long sum(long long a, long long b) {
+	return (a + b) % mod;
+}
+
+long long sub(long long a, long long b) {
+	return (a - b) % mod;
+}
+
+long long prod(long long a, long long b) {
+	return (a * b) % mod;
+}
+
+long long fpow(long long a, long long p) {
+	long long ret = 1;
+
+	while (p) {
+		if (p & 1) ret = prod(ret, a);
+		a = prod(a, a);
+		p >>= 1;
+	}
+
+	return ret;
+}
+
+long long divmod(long long a, long long b) {
+	return prod(a, fpow(b, mod-2));
+}
+
+
+long long xs[maxn];
+long long ways[maxx + 1];
+int n, x;
+
+int main() {
+	std::ios_base::sync_with_stdio(0);
+	std::cin.tie(0);
+	std::cin >> n >> x;
+	for (int i = 0; i < n; i++) {
+		std::cin >> xs[i];
+	}
+
+	ways[0] = 1;
+	long long ans = 0;
+	for (int i = 0; i <= x; i++) {
+		if (i + xs[0] > x) {
+			ans = sum(ans, prod(ways[i], divmod(1,n)));
+		}
+
+		for (int j = 0; j < n; j++) {
+			auto xi = xs[j];
+			if (i + xi <= maxx) {
+				ways[i+xi] = sum(ways[i+xi] , 
+						prod(ways[i], divmod(1, n)));
+			}
+		}
+	}
+
+	std::cout << ans << '\n';
+}
+
+// AC, probability, mod