[CF] 455A - Boredom | WFY's Blog

I thought of a different approach to the question, not sure if it will work.

My Solution

Idea: The final score that we get = sum - sum_deleted, which means that we are trying to minimize the sum of integers that we deleted.

Each time we choose a_k, we delete deleted[a_k] = count[a_{k-1}] * a_{k-1} + count[a_{k+1}] * a_{k+1}, where count[a_k] denotes the number of a_k. So is it possible to greedily search for the minimal deleted[a_k] each time and delete them? I am not sure if this will lead to optimal solution.

Official Solution

Use dynamic programming. For each integer a_k, we either choose it or delete it. So we can do dp from the largest integer that we have:

Suppose dp[k] denotes the largest score that we can have with the array of integers given and delete all the integers larger than k. So for test case

1 2	10 1 2 2 3 3 3 4 4 4 4

dp[2] would denote the largest score that we can get from array 1 2 2

dp[k] can be degraded to situation where either we choose k or not choose k. If we choose k, then k- would also be deleted, which would be dp[k-2] + cnt[k] * k. If we don’t choose k, then it would be the same as dp[k-1]. So the dp formula:

dp[k] = max({dp[k-2] + cnt[k] * k, dp[k-1]})

#include <iostream>
#include <cstring>
#include <algorithm>
using namespace std;
long long dp[100001] = {0};
int cnt[100001] = {0};
int n;
int max_num = 0;

int main()
{
    scanf("%d", &n);
    for (int i = 0; i < n; i++) {
        int a;
        scanf("%d", &a);
        cnt[a]++;
        max_num = max(a,max_num);
    }
    dp[1] = cnt[1];
    for (int i = 2; i <= max_num; i++) {
        dp[i] = max({dp[i-1], dp[i-2] + (long long)cnt[i] * i});
    }
    printf("%I64d", dp[max_num]);
    return 0;
}

Thoughts

I didn’t thought about dynamic programming ways to solve this problem. I guess the best way to come up with a dp solution is to degrade the question to easier scenarios. I think this question is actually a typical “choose or not” type of dp problem.

Also, still not sure whether my solution will work. I will think about it when I am better :D