【NOI Online 3】Kettle

題目解法

顯然答案應爲長度爲 $k+1$ 的區間和的最大值，用前綴和 $+$ 差分計算即可。

時間複雜度 $O(N)$ 。

#include<bits/stdc++.h>
using namespace std;
const int MAXN = 1e6 + 5;
typedef long long ll;
template <typename T> void chkmax(T &x, T y) {x = max(x, y); }
template <typename T> void chkmin(T &x, T y) {x = min(x, y); } 
template <typename T> void read(T &x) {
	x = 0; int f = 1;
	char c = getchar();
	for (; !isdigit(c); c = getchar()) if (c == '-') f = -f;
	for (; isdigit(c); c = getchar()) x = x * 10 + c - '0';
	x *= f;
}
int n, k, a[MAXN], s[MAXN];
int main() {
	freopen("kettle.in", "r", stdin);
	freopen("kettle.out", "w", stdout);
	read(n), read(k);
	for (int i = 1; i <= n; i++)
		read(a[i]), s[i] = s[i - 1] + a[i];
	int Max = 0;
	for (int i = k + 1, j = 0; i <= n; i++, j++)
		chkmax(Max, s[i] - s[j]);
	cout << Max << endl;
	return 0;
}