【题解】CF760B：Frodo and pillows

原题传送门
还是挺好做的吧
首先可以想到二分答案
想一想枕头个数如何分配最好，自然是呈现金字塔形式分配
即k号床最多，旁边两个床少一个，再旁边少两个……
check就是直接数学计算一下，用等差数列求和，注意一下细节处理
Code：

#include <bits/stdc++.h>
#define LL long long
using namespace std;
LL n, m, k;

bool check(LL mid){
	LL x = k + mid - 1, sum = 0;
	if (x > n) sum += (mid + mid - n + k) * (n - k + 1) / 2; else
	sum += (mid + 1) * mid / 2 + n - k - mid + 1;
	x = k - mid + 1;
	if (x < 1) sum += (mid + mid - k) * (k - 1) / 2; else
	sum += (mid - 1) * mid / 2 + k - mid;
	return sum <= m;
}

int main(){
	scanf("%lld%lld%lld", &n, &m, &k);
	LL l = 1, r = m, ans;
	while (l <= r){
		LL mid = (l + r) >> 1;
		if (check(mid)) ans = mid, l = mid + 1; else r = mid - 1;
	} 
	printf("%lld\n", ans);
	return 0;
}