Codeforces 362B Petya and staircases

Test instructions: a little boy is going up the stairs, he can walk 1 steps or 2 steps or 3 steps at a time, but some of the steps are dirty and he doesn't want to walk on the dirty steps. With a total of n steps and M dirty steps, he started on the 1th step and went to the nth step. Ask the boy if he can walk to n steps without stepping on the dirty steps.

Idea: For a given m-dirty sequence, after sorting first, there is no contiguous three numbers on the line.

 
 
 1 #include<cstdio>
 2 #include<cstring>
 3 #include<algorithm>
 4 #include<iostream>
 5 #include<cstdlib>
 6 #include<string>
 7 #include<cmath>
 8 #include<vector>
 9 using namespace std;
10 const int maxn=1e5+7;
11 const double eps=1e-8;
12 const double pi=acos(-1);
13 #define ll long long
14 #define clc(a,b) memset(a,b,sizeof(a))
15 
16 int main()
17 {
18     ll n;
19     int m;
20     int a[3010];
21     scanf("%I64d%d",&n,&m);
22     clc(a,0);
23     for(int i=1; i<=m; i++)
24         scanf("%d",&a[i]);
25     sort(a+1,a+1+m);
26     if(a[1]==1||a[m]==n)
27     {
28         printf("NO\n");
29     }
30     else
31     {
32         int flag=0;
33         for(int i=1; i<=m-2; i++)
34         {
35             int j=a[i];
36             if(j+1==a[i+1]&&j+2==a[i+2])
37             {
38                 flag=1;
39                 break;
40             }
41         }
42         if(flag)
43             printf("NO\n");
44         else
45             printf("YES\n");
46     }
47     return 0;
48 }

Codeforces 362B Petya and staircases