acwing3418. 杨辉三角形

题目

题解

#include<iostream>
using namespace std;
typedef long long LL;
int n;
LL C(int a,int b){
    LL res=1;
    for(int i=a,j=1;j<=b;j++,i--){
        res=res*i/j;
        if(res>n)return res;
    }
    return res;
}
bool check(int k){ 
    LL l = k * 2, r = max((LL)n, l);
    while(l<r){
        LL mid=(l+r)>>1;
        if(C(mid,k)>=n)r=mid;
        else l=mid+1;
    }
    if(C(r,k)!=n)return false;
    cout<<r*(r+1)/2+k+1<<endl;
    return true;
}
int main(){
    cin>>n;
    for(int k=16;k>=0;k--){
        if(check(k)){
            break;
        }
    }
    return 0;
}