[CF954G]Castle Defense

题目大意：有$n$个点，每个点最开始有$a_i$个弓箭手，在第$i$个位置的弓箭手可以给$[i-r,i+r]$区间加上$1$的防御，你还有$k$个弓箭手，要求你最大化最小防御值

题解：二分答案，从右向左扫，显然得知，给一个位置增加防御值最好方式是在最右段加一些弓箭手。

卡点：无

 

C++ Code：

#include <cstdio>
#define maxn 500010
const long long inf = 0x3f3f3f3f3f3f3f3f;
int n, R;
long long k, protect1[maxn], s[maxn], protect2[maxn], protect3[maxn], p[maxn], protect4[maxn];
inline int min(int a, int b) {return a < b ? a : b;}
inline bool check(long long mid) {
	__builtin_memset(p, 0, sizeof p);
	int l = -R, r = R;
	long long now = 0, K = k;
	for (int i = 0; i < r; i++) now += s[i];
	for (int i = 0; i < n; i++) {
		now += s[r], now -= s[l - 1] + p[l - 1];
		if (now < mid) {
			p[r] = mid - now;
			K -= p[r];
			if (K < 0) return false;
			now = mid;
		}
		r++, l++;
	}
	return true;
}
int main() {
	scanf("%d%d%lld", &n, &R, &k);
	for (int i = 0; i < n; i++) scanf("%lld", s + i);
	long long l = 0, r = inf, ans;
	while (l <= r) {
		long long mid = l + r >> 1;
		if (check(mid)) {
			l = mid + 1;
			ans = mid;
		} else r = mid - 1;
	}
	printf("%lld\n", ans);
	return 0;
}