cf 760B.Frodo and pillows
二分，判断条件就是最小情况（设当前k位取x）比剩余值（m-x）要小。（貌似又做麻烦了2333）
1 #include<bits/stdc++.h> 2 #define LL long long 3 #define N 100005 4 using namespace std; 5 inline int ra() 6 { 7 int x=0,f=1; char ch=getchar(); 8 while (ch<'0' || ch>'9') {if (ch=='-') f=-1; ch=getchar();} 9 while (ch>='0' && ch<='9') {x=x*10+ch-'0'; ch=getchar();} 10 return x*f; 11 } 12 int n,m,k; 13 bool check(LL x) 14 { 15 if (x==1) return m>=n; 16 LL mx1=(k-1)*(x+1+x+k-1)>>1,mn1; 17 LL mx2=(n-k)*(x+1+x+n-k)>>1,mn2; 18 if (x>=k) mn1=(k-1)*(x+x-k)>>1; 19 else mn1=((x-1)*(x)>>1)+(k-x); 20 if (x>=n-k) mn2=(n-k)*(x-1+x-n+k)>>1; 21 else mn2=((x-1)*(x)>>1)+(n-k-x+1); 22 if (mn1+mn2>m-x) return 0; 23 return 1; 24 } 25 int main() 26 { 27 n=ra(); m=ra(); k=ra(); 28 int l=0,r=m; 29 // check(3); while (1); 30 while (l<r) 31 { 32 int mid=l+r>>1; 33 if (check(mid)) l=mid+1; 34 else r=mid; 35 } 36 if (check(l)) cout<<l; else cout<<l-1; 37 return 0; 38 }
