ACdream 1080 面面数

http://acdreamoj.sinaapp.com/problem.php?id=1080

递推公式  自己没有弄出来   记住吧  

#include<iostream>
#include<stdio.h>
#include<cstring>
#include<algorithm>
using namespace std;

long long N;

long long find( int sta,int end,long long val )
{
    if( end - sta <= 4 )
    {
        for( long long i = sta; i <= end; i++ )
        if( i*i-i+2 >= val )
           return i;
        return end;
    }
    long long mid = ( sta + end )>>1;
    if( mid*mid-mid+2 >= val )
         return find( sta,mid,val );
    else return find( mid+1,end,val );
}

int main( )
{
     int T,res;scanf("%d",&T);
     while( T-- )
     {
        scanf("%I64d",&N);
        if( N == 1 )
             res = 0;
        else res = find( 1,200000,N );
        printf("%d\n",res);
     }
     return 0;
}