AGC 030 B - Tree Burning

B - Tree Burning

链接

题意：

　　一个长度为L的环，有n个位置上有树，从0出发，每次选择一个方向（顺时针或者逆时针），一直走，直到走到一棵树的位置，烧掉这棵树，重复这个过程，直到没有树。求最多走多少距离。

分析：

　　最优一定是LLLRLRLRL……类似这样的，于是枚举每个点，计算答案。

代码：

#include<cstdio>
#include<algorithm>
#include<cstring>
#include<iostream>
#include<cmath>
#include<cctype>
#include<set>
#include<queue>
#include<vector>
#include<map>
using namespace std;
typedef long long LL;

inline int read() {
    int x=0,f=1;char ch=getchar();for(;!isdigit(ch);ch=getchar())if(ch=='-')f=-1;
    for(;isdigit(ch);ch=getchar())x=x*10+ch-'0';return x*f;
}

const int N = 200005;

LL s1[N], s2[N], a[N], L, Ans;
int n;

LL f(int l,int r) {
    return s1[r] - a[l] * (r - l) - s1[l];
}

void Calc() {
    for (int i = 1; i <= n; ++i) 
        s1[i] = s1[i - 1] + a[i], s2[i] = s2[i - 1] + (L - a[n - i + 1]);
    for (int i = 0; i < n; ++i) {
        LL now = a[i] * (n - i) + a[i];
        int cnt = (n - i) / 2;
        if ((n - i) % 2 == 0) 
            now += s2[cnt] * 2 + f(i, i + cnt) * 2 - (a[i + cnt] - a[i]);
        else 
            now += s2[cnt + 1] * 2 + f(i, i + cnt) * 2 - (L - a[n - cnt]);
        Ans = max(Ans, now);
    }
}
int main() {
    L = read(); n = read();
    for (int i = 1; i <= n; ++i) a[i] = read();
    Calc();
    for (int i = 1; i <= n; ++i) a[i] = L - a[i];
    reverse(a + 1, a + n + 1);
    Calc();
    cout << Ans;
    return 0;
}