Codeforces 1032C. Playing Piano

标签是dp，可是想了很久也想不出状态转移方程，还是贪心可搞。贪心的关键在于判断下一步，不如到第i个的时候，我们不仅要比较a[i]和a[i+1]，还要比较a[i+1]和a[i+2]以确定正确的最优的选择。

#include <bits/stdc++.h>
using namespace std;
int n,a[201010],cur; 
vector<int> ans;
int main(){
	ios::sync_with_stdio(false);
	cin.tie(0);cout.tie(0);
	cin>>n;
	for(int i =0;i<n;i++) cin>>a[i];
	if(a[0] == a[1]) ans.push_back(3);
	else if(a[0] > a[1]) ans.push_back(5);
	else ans.push_back(1);
	for(int i = 0;i<n-1;i++){
		if(a[i]<a[i+1]&&ans[i]==5){
			cout<<-1;
			return 0;
		} 
		if(a[i]>a[i+1]&&ans[i]==1){
			cout<<-1;
			return 0;
		}
		if(a[i+1]>a[i]) cur = (a[i+2]<a[i+1]&&i<n-2?5:ans[i]+1);
		else if(a[i+1]<a[i]) cur = (a[i+2]>a[i+1]&&i<n-2?1:ans[i]-1);
		else {
			cur = (ans[i]==3?2:3);
			if(i<n-2) {
				if(a[i+2]>a[i+1]) cur = (ans[i]==1?2:1);
				else if(a[i+2]<a[i+1]) cur = (ans[i] ==5?4:5);
			}  
		}
		ans.push_back(cur); 
	}
	for(int i = 0;i<ans.size();i++)	cout<<ans[i]<<' ';
	return 0;
}