poj1759（二分找寻最值）

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/*translation:有n个灯泡挂在距离地面分别是hi的高度上。对每个hi有如下几个条件的限制:H1 = A ;Hi = (H i-1 + H i+1)/2 - 1, for all 1 < i < N ;HN = B ;Hi >= 0, for all 1 <= i <= N 求最大的B值是多少solution:二分枚举第二个灯泡的高度即可note:#：输出时记得输出%f而不是%lf，迷之贡献WA。。。date:2016.11.8*/#include <iostream>#include <cstdio>#include <cstring>using namespace std;const int maxn = 1000 + 20;double h[maxn], ans, a;int n;bool check(double mid){h[1] = mid;for(int i = 2; i < n; i++){h[i] = 2 * h[i-1] + 2 - h[i-2];if(h[i] < 1e-8)return false;}ans = h[n-1];return true;}int main(){//freopen("in.txt", "r", stdin);    while(~scanf("%d%lf", &n, &a)){h[0] = a;double lb = 0, ub = 1000 + 16;for(int i = 0; i < 100; i++){double mid = (lb + ub) / 2;if(check(mid))ub = mid;elselb = mid;}printf("%.2f\n", ans);    }    return 0;}

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