[BZOJ1783] [Usaco2010 Jan]Taking Turns

题目描述

Description

Farmer John has invented a new way of feeding his cows. He lays out N (1 <= N <= 700,000) hay bales conveniently numbered 1..N in a long line in the barn. Hay bale i has weight W_i (1 <= W_i <= 2,000,000,000). A sequence of six weights might look something like: 17 5 9 10 3 8 A pair of cows named Bessie and Dessie walks down this line after examining all the haybales to learn their weights. Bessie is the first chooser. They take turns picking haybales to eat as they walk (once a haybale is skipped, they cannot return to it). For instance, if cows Bessie and Dessie go down the line, a possible scenario is: * Bessie picks the weight 17 haybale * Dessie skips the weight 5 haybale and picks the weight 9 haybale * Bessie picks the weight 10 haybale * Dessie skips the weight 3 haybale and picks the weight 8 haybale Diagrammatically: Bessie | | 17 5 9 10 3 8 Dessie | | This scenario only shows a single skipped bale; either cow can skip as many as she pleases when it's her turn.Each cow wishes to maximize the total weight of hay she herself consumes (and each knows that the other cow has this goal).Furthermore, a cow will choose to eat the first bale of hay thatmaximimizes her total weight consumed. Given a sequence of hay weights, determine the amount of hay that a pair of cows will eat as they go down the line of hay. 一排数,两个人轮流取数,保证取的位置递增,每个人要使自己取的数的和尽量大,求两个人都在最优策略下取的和各是多少。

Input

Line 1: A single integer: N * Lines 2..N+1: Line i+1 contains a single integer: W_i

Output

Line 1: Two space-separated integers, the total weight of hay consumed by Bessie and Dessie respectively

Sample Input

6
17
5
9
10
3
8

Sample Output

27 17

题目分析

思路很巧妙啊
表示从i开始选 先手最大的和是多少 表示从i开始选 最优情况下 下一个选的位置是啥
那么从后往前推 根据是不是选a[i]分类讨论一下就好了
注意 对手也有最优策略

#include <cstdio>
#include <cstring>
#include <set>
#include <map>
#include <vector>
#include <cmath>
#include <queue>
#include <algorithm>
using namespace std;
int n;
long long a[700100],f[700100];
int id[700100];
int main()
{   
    scanf("%d",&n);
    for(int i=1;i<=n;i++) scanf("%lld",&a[i]);
    f[n]=a[n],id[n]=n;
    for(int i=n-1;i>=1;i--)
    {
        f[i]=f[i+1],id[i]=id[i+1];
        if(a[i]+f[id[i+1]+1]>=f[i]) f[i]=a[i]+f[id[i+1]+1],id[i]=i;
    }
    printf("%lld %lld",f[1],f[id[1]+1]);
    return 0;
}


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