Xiao Sha's pain (double pointer

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#include<bits/stdc++.h>
using namespace std;
int n,p;
#define int long long
const int N=1e6+9;
int arr[N];
signed main(){
     
    cin>>n>>p;
    for(int i=1;i<=n;i++)scanf("%lld",&arr[i]),arr[i]%=p;
    sort(arr+1,arr+1+n);
    int mx=(arr[n-1]+arr[n])%p;// Note that there mx It is possible that the maximum value will not be obtained   For example, when the data are all equal 
     for(int l=1,r=n;l<=n;l++){
    
        while(r>0&&arr[r]+arr[l]>=p){
    
            r--;
        }
        if(r>l)mx=max(mx,arr[r]+arr[l]);// The same array must r>l
    }
    cout<<mx<<endl;
    return 0;
}