Codeforces Round #172 (Div. 2) Maximum Xor Secondary

Problem: Maximum Xor Secondary

///     Raihan Ruhin
///     CSE, Jahangirnagar University.
///     Dhaka-Bangladesh.
///     id: raihanruhin (topcoder / codeforces / codechef / hackerrank / uva / uvalive / spoj), 3235 (lightoj)
///     mail: raihanruhin@ (yahoo / gmail / facebook)
///     blog: ruhinraihan.blogspot.com

#include<bits/stdc++.h>
using namespace std;

#define SET(a) memset(a,-1,sizeof(a))
#define CLR(a) memset(a,0,sizeof(a))
#define PI acos(-1.0)

#define MOD 1000000007
#define MX 100010

long long arr[MX];
int main()
{
    ios_base::sync_with_stdio(0);cin.tie(0);
    int tc,kk=1, n;
    cin>>n;
    for(int i=0;i<n;i++) cin>>arr[i];
    //forward increasing.
    stack<long long>fi;
    fi.push(arr[0]);
    fi.push(arr[1]);
    long long mx=arr[0]^arr[1];
    for(int i=2;i<n;i++)
    {
        while(!fi.empty() && fi.top()<arr[i])
            fi.pop();
        fi.push(arr[i]);
        long long last=fi.top();
        fi.pop();
        if(!fi.empty())
        {
            long long prev=fi.top();
            mx=max(mx, prev^last);
        }
        fi.push(last);
    }
    //backward increasing
    stack<long long>bi;
    bi.push(arr[n-1]);
    bi.push(arr[n-2]);
    mx=max(mx, arr[n-1]^arr[n-2]);
    for(int i=n-3;i>=0;i--)
    {
        while(!bi.empty() && bi.top()<arr[i])
            bi.pop();
        bi.push(arr[i]);
        long long last=bi.top();
        bi.pop();
        if(!bi.empty())
        {
            long long prev=bi.top();
            mx=max(mx, prev^last);
        }
        bi.push(last);
    }

    cout<<mx;
    return 0;
}