Codeforces Round #262 (Div. 2) 460C. Present（二分）

题目链接：http://codeforces.com/problemset/problem/460/C

Little beaver is a beginner programmer, so informatics is his favorite subject. Soon his informatics teacher is going to have a birthday and the beaver has decided to prepare a present for her. He planted n flowers in a row on his windowsill and started waiting for them to grow. However, after some time the beaver noticed that the flowers stopped growing. The beaver thinks it is bad manners to present little flowers. So he decided to come up with some solutions.

There are m days left to the birthday. The height of the i-th flower (assume that the flowers in the row are numbered from 1 to n from left to right) is equal to ai at the moment. At each of the remaining m days the beaver can take a special watering and water w contiguous flowers (he can do that only once at a day). At that each watered flower grows by one height unit on that day. The beaver wants the height of the smallest flower be as large as possible in the end. What maximum height of the smallest flower can he get?

The first line contains space-separated integers n, m and w (1 ≤ w ≤ n ≤ 105; 1 ≤ m ≤ 105). The second line contains space-separated integers a1, a2, ..., an (1 ≤ ai ≤ 109).

Print a single integer — the maximum final height of the smallest flower.

6 2 3
2 2 2 2 1 1

2

2 5 1
5 8

9

In the first sample beaver can water the last 3 flowers at the first day. On the next day he may not to water flowers at all. In the end he will get the following heights: [2, 2, 2, 3, 2, 2]. The smallest flower has height equal to 2. It's impossible to get height 3 in this test.

题意：

给出N朵花的初始的高度，从左到右排列，最多浇水m天，每天只能浇一次，每次可以使连续的 w 朵花的高度增加单位长度1，问最后m天浇完水后最矮的花的高度最高是达到多少。

思路：

从最低和最高（记得+m）的高度之间二分枚举高度，找出最大能适合的！见代码……

代码如下：

#include 
#include 
#include 
using namespace std;
#define LL __int64
const int MAXN = 200017;
LL a[MAXN], b[MAXN], v[MAXN];
int main()
{LL n, m, w;while(~scanf("%I64d %I64d %I64d",&n,&m,&w)){LL low = 1e9, top = -1;for(int i = 1 ; i <= n ; i++){scanf("%I64d", &a[i]);if(a[i] < low)low = a[i];if(a[i] > top)top = a[i];}top += m;//最大的高度LL mid, ans = -1 ;while(low <= top){mid = (low + top)>>1 ;for(int i = 1 ; i <= n ; i++)b[i] = max(mid - a[i],(LL)0);//每朵花需要浇水的天数memset(v,0,sizeof(v));LL day = m;//天数LL c = 0;//已经浇了的天数for(int i = 1; i <= n; i++){c += v[i];b[i] -= c;//已浇c天if(b[i] > 0){day -= b[i];if(day < 0)//天数不够break;c += b[i];//已浇b[i]天v[i+w] -= b[i];//浇水到这里b[i] = 0;}}if(day < 0)//不符合，向更小的值二分寻找top = mid - 1;else//继续向更大的值二分寻找{ans = mid;low = mid + 1;}}printf("%I64d\n", ans);}return 0;
}

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