HELP -SPOJ-MAXIMUM SUM-KGSS

help
1

Getting a segmentation fault.Using Segment tree.

Problem link- SPOJ.com - Problem KGSS

#include<iostream>
#include<bits/stdc++.h>

using namespace std;

#define ll long long
//#define vi vector<>

void debugMode() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    #ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
    #endif // ONLINE_JUDGE
}



void build(vector<ll> &arr,vector<pair<ll,ll>> &tree,int s ,int e,int tidx)
{
    if(s==e)
    {
        tree[tidx].first=arr[s];
        tree[tidx].second = 0;
        return;
    }
    int mid=(s+e)/2;
    build(arr,tree,s,mid,2*tidx+1);
    build(arr,tree,mid+1,e,2*tidx+2);
    tree[tidx].first = max(tree[2*tidx+1].first,tree[2*tidx+2].first);
    tree[tidx].second = min(max(tree[2*tidx+1].second,tree[2*tidx+2].first),max(tree[2*tidx+1].first,tree[2*tidx+2].second));

  
}

void update(vector<ll> &arr,vector<pair<ll,ll>> &tree,int s,int e,int tidx,int x,int val)
{
    if(s==e)
    {
        arr[x]=val;
        tree[tidx].first=val;
    }
    int mid=(s+e)/2;
    if(x>mid)
    update(arr,tree,mid+1,e,2*tidx+2,x,val);
     else
    update(arr,tree,s,mid,2*tidx+1,x,val);
    

    //self-work
   
    tree[tidx].first = max(tree[2*tidx+1].first,tree[2*tidx+2].first);
    tree[tidx].second = min(max(tree[2*tidx+1].second,tree[2*tidx+2].first),max(tree[2*tidx+1].first,tree[2*tidx+2].second));


}

pair<ll,ll> query(vector<pair<ll,ll>> &tree,int s,int e,int tidx,int l,int r)
{
    if(s>r or e<l){pair<ll,ll> pr(0,0); return pr ;}
    if (s>=l and e<=r)
    return tree[tidx];
    int mid = (s+e)/2;
    pair<ll,ll> ansl = query(tree,s,mid,2*tidx+1,l,r);
    pair<ll,ll> ansr = query(tree,mid+1,e,2*tidx+2,l,r);
    pair<ll,ll> ans;
    ans.first = max(ansl.first,ansr.first);
    ans.second = max(ansl.second,ansr.second);
    return ans;
}

int main()
{
    debugMode();
    
    int n,q;
    cin>>n;
    vector<ll> arr(n);
    for(int i=0;i<n;i++) { int m;cin>>m;arr.push_back(m);}
    vector<pair<ll,ll>> segtree(4*n);
    cin>>q;
    build(arr,segtree,0,n-1,0);
    while(q--)
    {
        char type;
        cin>>type;
        if(type=='U')
        {
            int val;
            int x;
            cin>>x>>val;
            update(arr,segtree,0,n-1,0,x-1,val);
        }
        else
        {
            int l,r;
            cin>>l>>r;
            //pair<ll,ll> sum;
            
            auto sum = query(segtree,0,n-1,0,l-1,r-1);
            cout<<sum.first+sum.second<<"\n";
        }
    }
    return 0;
}