AtCoder：Widespread（二分）

D - Widespread

---

Time limit : 2sec / Memory limit : 256MB

Score : 400 points

Problem Statement

You are going out for a walk, when you suddenly encounter N monsters. Each monster has a parameter called health, and the health of the i-th monster is hi at the moment of encounter. A monster will vanish immediately when its health drops to 0 or below.

Fortunately, you are a skilled magician, capable of causing explosions that damage monsters. In one explosion, you can damage monsters as follows:

• Select an alive monster, and cause an explosion centered at that monster. The health of the monster at the center of the explosion will decrease by A, and the health of each of the other monsters will decrease by B. Here, A and B are predetermined parameters, and A>B holds.

At least how many explosions do you need to cause in order to vanish all the monsters?

Constraints

• All input values are integers.
• 1≤N≤105
• 1≤B<A≤109
• 1≤hi≤109

---

Input

Input is given from Standard Input in the following format:

N A Bh1h2:hN

Output

Print the minimum number of explosions that needs to be caused in order to vanish all the monsters.

---

Sample Input 1

Copy

4 5 38742

Sample Output 1

Copy

2

You can vanish all the monsters in two explosion, as follows:

• First, cause an explosion centered at the monster with 8 health. The healths of the four monsters become 3, 4, 1 and −1, respectively, and the last monster vanishes.
• Second, cause an explosion centered at the monster with 4 health remaining. The healths of the three remaining monsters become 0, −1 and −2, respectively, and all the monsters are now vanished.

---

Sample Input 2

Copy

2 10 42020

Sample Output 2

Copy

4

You need to cause two explosions centered at each monster, for a total of four.

---

Sample Input 3

Copy

5 2 1900000000900000000100000000010000000001000000000

Sample Output 3

Copy

800000000

题意：有N只怪物和攻击力A，B，每次攻击可选择对其中一只怪物造成A伤害，同时对其余怪物造成B伤害，问最小的攻击次数杀光怪物。

思路：二分答案即可，每只怪物减去x*b血，如果还没死就再砍blood/(A-B)次。

# include <bits/stdc++.h>using namespace std;typedef long long LL;const int maxn = 1e5;int n, a, b, m[maxn+3];bool judge(LL x){    LL sum = 0;    for(int i=0; i<n; ++i)    {        LL t = (LL)m[i]-x*b;        if(t>0)        {            sum += (int)ceil(t*1.0/(a-b));            if(sum > x || sum < 0)                return false;        }    }    return true;}int main(){    while(~scanf("%d%d%d",&n,&a,&b))    {        int imax = 0;        for(int i=0; i<n; ++i) scanf("%d",&m[i]), imax = max(imax, m[i]);        LL l=0, r=imax;        while(l<r)        {            LL mid = l+r>>1;            if(judge(mid)) r=mid;            else l=mid+1;        }        printf("%lld\n",r);    }    return 0;}