Codeforces 760B Frodo and pillows

题目链接：http://codeforces.com/problemset/problem/760/B

题意：n个床位，m个枕头，第k个位置最多有多少个枕头，其中相邻之间的差<=1;

第k个位置最大，那么两边只能是-1，-2，-3下走。

二分查找。

#include <bits/stdc++.h>

using namespace std;

long long n,m,k;

int Slove(long long mid) {
    long long sum = 0;
    if(k>=mid)
    {
        sum+=(1+mid)*mid/2;
        sum+=(k-mid);
    }
    if(k<mid) {
        sum+=((mid-k+1)+mid)*k/2;
    }

    if(k+(mid-1)<=n) {
        sum+=(1+mid)*mid/2;
        sum+=n-(k+(mid-1));
    }

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

int main()
{
    //scanf("%d%d%d",&n,&m,&k);
    cin>>n>>m>>k;
    long long ans = 0;
    long long l = 1;
    long long r = m;
    while(l<=r) {

        long long mid = (l+r)/2;
        if(Slove(mid)==1) {
            ans = mid;
            l = mid+1;
        }
        else r = mid-1;
    }
    cout<<ans<<endl;
    return 0;
}