sorting, two pointers(cf div.3 1113)

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 int n,a[200005];
 4 int main()
 5 {
 6     scanf("%d",&n);
 7     for(int i=1;i<=n;i++)scanf("%d",&a[i]);
 8     sort(a+1,a+n+1);
 9     int j=n,ans=0;
10     for(int i=n;i>=1;i--){
11         while(a[i]-a[j]<=5 && j>=1)
12             if(a[i]-a[j]<=5){
13                 j--;
14                 ans=max(i-j,ans);}
15     }
16     printf("%d\n",ans);
17 }