POJ1759 Garland —— 二分

 1 #include <iostream>
 2 #include <cstdio>
 3 #include <cstring>
 4 #include <algorithm>
 5 #include <vector>
 6 #include <cmath>
 7 #include <queue>
 8 #include <stack>
 9 #include <map>
10 #include <string>
11 #include <set>
12 #define rep(i,a,n) for(int (i) = a; (i)<=(n); (i)++)
13 #define ms(a,b) memset((a),(b),sizeof((a)))
14 using namespace std;
15 typedef long long LL;
16 const double EPS = 1e-8;
17 const int INF = 2e9;
18 const LL LNF = 9e18;
19 const int mod = 1e9+7;
20 const int maxn = 1e5+10;
21 
22 int n;
23 double A, ans;
24 
25 bool test(double x1, double x2)
26 {
27     for(int i = 3; i<=n; i++)   //递推出每个点的高度
28     {
29         double x3 = 2*x2+2-x1;
30         if(x3<=0) return false; //出现负数，证明接地了， 不符合。
31         x1 = x2, x2 = x3;
32     }
33     ans = x2;   //符合条件， 则更新答案。
34     return true;
35 }
36 
37 int main()
38 {
39     while(scanf("%d%lf", &n, &A)!=EOF)
40     {
41         double l = 0, r = A;    //二分第二个点
42         while(l+EPS<=r)
43         {
44             double mid = (l+r)/2;
45             if(test(A, mid))
46                 r = mid - EPS;
47             else
48                 l = mid + EPS;
49         }
50         printf("%.2f\n", ans);
51     }
52 }