URAL 1066 Garland 二分

二分H2的位置，判断条件为是否有Hi < 0

 1 #include <cstdio>

 2 #include <cstring>

 3 #include <cstdlib>

 4 #include <algorithm>

 5 #include <cmath>

 6 

 7 using namespace std;

 8 

 9 const double eps = 1e-9;

10 

11 int N;

12 double A;

13 

14 int dcmp( double a )

15 {

16     if ( fabs(a) < eps ) return 0;

17     return a < 0 ? -1 : 1;

18 }

19 

20 bool cal( double mid, double &ans )

21 {

22     double Hi2 = A, Hi1 = mid, Hi0;

23     for ( int i = 0; i < N - 2; ++i )

24     {

25         Hi0 = 2.0 * ( Hi1 + 1.0 ) - Hi2;

26         if ( dcmp( Hi0 ) < 0 ) return false;

27         Hi2 = Hi1, Hi1 = Hi0;

28     }

29     ans = Hi0;

30     return true;

31 }

32 

33 int main()

34 {

35     while ( ~scanf( "%d%lf", &N, &A ) )

36     {

37         double l = 0.0, r = A;

38         //int cnt = 0;

39         double ans;

40         while ( dcmp( r - l ) > 0 )

41         {

42             //++cnt;

43             double mid = ( l + r ) / 2.0;

44             //printf( "%f %f %f\n", l, r, mid );

45             double tmp;

46             if ( !cal( mid, tmp ) ) l = mid;

47             else

48             {

49                 r = mid;

50                 ans = tmp;

51             }

52             //if ( cnt > 100 ) break;

53         }

54         printf( "%.2f\n", ans );

55     }

56     return 0;

57 }