Educational Codeforces Round 63 (Rated for Div. 2) D

 dp[i][0] 表示之前不选的最大值，dp[i][1]表示选这个的最大值，dp[i][2]表示现在以后不选的最大值

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int maxn = 3e5+10;
ll a[maxn], dp[maxn][5];
int n,x;
ll ans;
int main()
{
	scanf("%d%d",&n,&x);
	for (int i = 1; i <= n; i++) scanf("%lld", &a[i]);
	for (int i = 1; i <= n; i++)
	{
		dp[i][0] = max(dp[i - 1][0] + a[i], a[i]);
		dp[i][1] = max(dp[i - 1][1] + a[i] * x, max(dp[i - 1][0]+a[i]*x,a[i]*x));
		dp[i][2] = max(dp[i - 1][2] + a[i],max(dp[i - 1][1] + a[i], a[i]));
		if(ans<max( dp[i][0],max(dp[i][1],dp[i][2])))
		{
			ans=max(dp[i][0],max(dp[i][1],dp[i][2]));
		}
	}
	printf("%lld\n", ans);
	return 0;
}