Codeforces 671B Robin Hood 二分答案

B. Robin Hood
time limit per test
1 second
memory limit per test
256 megabytes
input
standard input
output
standard output

We all know the impressive story of Robin Hood. Robin Hood uses his archery skills and his wits to steal the money from rich, and return it to the poor.

There are n citizens in Kekoland, each person has ci coins. Each day, Robin Hood will take exactly 1 coin from the richest person in the city and he will give it to the poorest person (poorest person right after taking richest’s 1 coin). In case the choice is not unique, he will select one among them at random. Sadly, Robin Hood is old and want to retire in k days. He decided to spend these last days with helping poor people.

After taking his money are taken by Robin Hood richest person may become poorest person as well, and it might even happen that Robin Hood will give his money back. For example if all people have same number of coins, then next day they will have same number of coins too.

Your task is to find the difference between richest and poorest persons wealth after k days. Note that the choosing at random among richest and poorest doesn’t affect the answer.
Input

The first line of the input contains two integers n and k (1 ≤ n ≤ 500 000, 0 ≤ k ≤ 109) — the number of citizens in Kekoland and the number of days left till Robin Hood’s retirement.

The second line contains n integers, the i-th of them is ci (1 ≤ ci ≤ 109) — initial wealth of the i-th person.
Output

Print a single line containing the difference between richest and poorest peoples wealth.
Examples
Input

4 1
1 1 4 2

Output

2

Input

3 1
2 2 2

Output

0

Note

Lets look at how wealth changes through day in the first sample.

[1, 1, 4, 2]
[2, 1, 3, 2] or [1, 2, 3, 2] 

So the answer is 3 - 1 = 2

In second sample wealth will remain the same for each person.

题意：
通过k次把最大数值减一加到最小数上，问最后最大数和最小数的差值最小是多少？
思路：
二分找出最大值最小可能的数，最小值最大可能的数，那么就可以使得差值最小。

#include<iostream>
#include<cstdio>
#include<cstring>
#include<string>
#include<algorithm>
#include<cmath>
#include<set>
using namespace std;
const int N=5e5+9;
typedef long long ll;
ll sum;
int a[N];
int n,k,pre;
bool ismin(int x)
{
    int t=k;
    int mid=lower_bound(a,a+n,x)-a;
    for(int i=0;i<mid;i++){
        t-=(x-a[i]);
        if(t<0)return 0;
    }
    return 1;
}
bool ismax(int x)
{
    int t=k;
    int mid=upper_bound(a,a+n,x)-a;
    for(int i=mid;i<n;i++){
        t-=(a[i]-x);
        if(t<0)return 0;
    }
    return 1;

}
int main()
{
    //freopen("f.txt","r",stdin);
    scanf("%d%d",&n,&k);
    sum=0;
    for(int i=0;i<n;i++)scanf("%d",&a[i]),sum+=a[i];
    sort(a,a+n);
    if(a[0]==a[n-1]){printf("0");return 0;}
    pre=sum/n;
    int l=pre,r=a[n-1];
    if(sum%n)l++;
    while(l<r){
        int m=l+(r-l)/2;
        if(ismax(m))r=m;
        else l=m+1;
    }
    int maxn=l;

    l=a[0],r=pre;
    while(l<r){
        int m=l+(r-l+1)/2;
        if(ismin(m))l=m;
        else r=m-1;
    }

    printf("%d\n",maxn-l);
    return 0;
}