UESTC 1015 Lweb and pepper --前，后缀最值

题意： n种食物，每种含花椒的概率为Pi，现在已经选择了[L,R]这个区间（下标）的食物，要再选一个，使总的食物只有一种含花椒的概率最大，问选哪个最好，相同的选下标小的。

解法： 就不写解法了。此处有官方题解： http://acm.uestc.edu.cn/bbs/read.php?tid=5835

维护一个前缀后缀的最值即可。

代码：

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <cmath>
#include <algorithm>
#define Mod 1000000007
#define eps 1e-8
using namespace std;
#define N 100002

int preMax[N],bacMax[N],preMin[N],bacMin[N];
double pMax[N],pMin[N],bMax[N],bMin[N],p[N];
int sgn(double x)
{
    if(x < -eps) return -1;
    if(x > eps)  return 1;
    return 0;
}

int main()
{
    int t,n,i,m,L,R;
    cin>>t;
    while(t--)
    {
        scanf("%d",&n);
        for(i=1;i<=n;i++)
            scanf("%lf",&p[i]);
        preMax[0] = 0;   preMin[0] = 0;
        pMax[0] = -1;      pMin[0] = Mod;
        bacMax[n+1] = n+1; bacMin[n+1] = n+1;
        bMax[n+1] = -1;  bMin[n+1] = Mod;
        for(i=1;i<=n;i++)
        {
            preMax[i] = preMax[i-1];
            preMin[i] = preMin[i-1];
            pMax[i] = pMax[i-1];
            pMin[i] = pMin[i-1];
            if(p[i] > pMax[i]) pMax[i] = p[i], preMax[i] = i;
            if(p[i] < pMin[i]) pMin[i] = p[i], preMin[i] = i;
        }
        for(i=n;i>=1;i--)
        {
            bacMax[i] = bacMax[i+1];
            bacMin[i] = bacMin[i+1];
            bMax[i] = bMax[i+1];
            bMin[i] = bMin[i+1];
            if(p[i] >= bMax[i]) bMax[i] = p[i], bacMax[i] = i;
            if(p[i] <= bMin[i]) bMin[i] = p[i], bacMin[i] = i;
        }
        scanf("%d",&m);
        while(m--)
        {
            scanf("%d%d",&L,&R);
            L++,R++;
            double B = 1.0, E = 0.0;
            for(i=L;i<=R;i++)
                B *= (1-p[i]), E += p[i]/(1-p[i]);
            if(sgn(1-E) > 0)
            {
                if(pMax[L-1] >= bMax[R+1]) printf("%d
",preMax[L-1]-1);
                else                       printf("%d
",bacMax[R+1]-1);
            }
            else
            {
                if(pMin[L-1] <= bMin[R+1]) printf("%d
",preMin[L-1]-1);
                else                       printf("%d
",bacMin[R+1]-1);
            }
        }
    }
    return 0;
}