洛谷 P2642 双子序列最大和

题目传送门

分别f[i][1/0]表示到第i位从前面或从后面子序列的最大和,最后枚举断点即可.

#include<iostream>
#include<cstdio>
#include<cstring>

using namespace std;

int n;
long long f[1000001][2],a[1000001],ans = -80000000000000,_ans[1000001][2];

inline long long _max(long long s,int d) {
	if(s > d) return s;
	return d;
}

int main() {
	scanf("%d",&n);
	for(int i = 1;i <= n; i++) {
		scanf("%lld",&a[i]);
		f[i][1] = f[i][0] = _ans[i][1] = _ans[i][0] = -8000000000000000;
	}
	_ans[1][0] = f[1][0] = a[1];
	for(int i = 2;i <= n; i++)
		f[i][0] = _max(f[i-1][0],0) + a[i],_ans[i][0] = max(_ans[i+1][0],f[i][0]);
	_ans[n][1] = f[n][1] = a[n];
	for(int i = n - 1;i >= 1; i--)
		f[i][1] = _max(f[i+1][1],0) + a[i],_ans[i][1] = max(_ans[i+1][1],f[i][1]);
	for(int i = 2;i < n; i++)
		ans = max(ans,_ans[i-1][0] + _ans[i+1][1]);
	printf("%lld",ans);
	return 0;
}