B. Marvolo Gaunt's Ring 前缀后缀

B. Marvolo Gaunt's Ring

这种一般只有三个的都可以处理前缀和后缀，再枚举中间这个值。

这个和之前写过的C. Four Segments 前缀后缀 处理方式很像。

#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <algorithm>
#include <queue>
#include <vector>
#include <iostream>
#include <string>
#define inf 0x3f3f3f3f
#define inf64 0x3f3f3f3f3f3f3f3f
using namespace std;
const int maxn = 1e5 + 10;
typedef long long ll;
ll a[maxn], pre[maxn], endss[maxn];
int main()
{
    ll n, p, q, r;
    cin >> n >> p >> q >> r;
    for (int i = 0; i <= n + 1; i++) endss[i] = pre[i] = -inf64;
    for (int i = 1; i <= n; i++) scanf("%lld", &a[i]);
    for (int i = 1; i <= n; i++) {
        pre[i] = max(pre[i - 1], a[i] * p);
    }
    for (int i = n; i >= 1; i--) {
        endss[i] = max(endss[i + 1], a[i] * r);
    }
    ll ans = -inf64;
    for (int i = 1; i <= n; i++) {
        ans = max(ans, pre[i] + endss[i] + q * a[i]);
        // printf("i=%d pre[%d]=%lld endss[%d]=%lld ans=%lld\n", i, i, pre[i], i, endss[i], ans);
    }
    printf("%lld\n", ans);
    return 0;
}