[洛谷P2642]双子序列最大和

#include<iostream>
#include<cstdio>
#include<algorithm>
#define LL long long
#define INF 2147483647
using namespace std;
const int MAXN = 1000000 + 50;
LL v[MAXN],minx[MAXN],ans = -INF,w[MAXN],hminx[MAXN];
int n,a[MAXN];
LL sum1[MAXN],sum2[MAXN];
int main(){
scanf("%d",&n);
for(int i = 1;i <= n;i ++){
scanf("%d",&a[i]);
v[i] = v[i - 1] + a[i];
minx[i] = min(minx[i - 1],v[i]);
}
sum1[1] = v[1];
for(int i = 1;i <= n;i ++){
if(i != 1)sum1[i] = max(sum1[i - 1],v[i] - minx[i - 1]);
w[n - i + 1] = w[n - i + 1 + 1] + a[n - i + 1];
hminx[n - i + 1] = min(hminx[n - i + 1 + 1],w[n - i + 1]);
}
sum2
= w
;
for(int i = n - 1;i >= 1;i --){
sum2[i] = max(sum2[i + 1],w[i] - hminx[i + 1]);
}
for(int i = 2;i < n;i ++){
ans = max(ans,sum1[i - 1] + sum2[i + 1]);
}
printf("%lld",ans);
return 0;
}