题解：dp
Description
Alex doesn‘t like boredom. That‘s why whenever he gets bored, he comes up with games. One long winter evening he came up with a game and decided to play it.
Given a sequence a consisting of n integers. The player can make several steps. In a single step he can choose an element of the sequence (let‘s denote it ak) and delete it, at that all elements equal to ak + 1 and ak - 1 also must be deleted from the sequence. That step brings ak points to the player.
Alex is a perfectionist, so he decided to get as many points as possible. Help him.
Input
The first line contains integer n (1 ≤ n ≤ 105) that shows how many numbers are in Alex‘s sequence.
The second line contains n integers a1, a2, ..., an (1 ≤ ai ≤ 105).
Output
Print a single integer — the maximum number of points that Alex can earn.
Sample Input
Input
2
1 2
Output
2
Input
3
1 2 3
Output
4
Input
9
1 2 1 3 2 2 2 2 3
Output
10
Hint
Consider the third test example. At first step we need to choose any element equal to 2. After that step our sequence looks like this [2, 2, 2, 2]. Then we do 4 steps, on each step we choose any element equals to 2. In total we earn 10 points.
1 #include<stdio.h>
2 #include<string.h>
3 #include<algorithm>
4 using namespace std;
5 typedef long long LL;
6 LL dp[100000],num[100000];
7 int main()
8 {
9 int n,i;
10 while(~scanf("%d",&n))
11 {
12 for(i=1;i<=n;i++)
13 {
14 int x;
15 scanf("%d",&x);
16 num[x]++;
17 }
18 for(i=2;i<=n;i++)
19 {
20 dp[i]=max(dp[i-1],dp[i-2]+(LL)num[i]*i);
21 }
22 printf("%lld\n",dp[n]);
23 }
24 return 0;
25 }