Problem name: Chefs in Queue

Problem link: Contest Page | CodeChef

I have solved it using the stack-based approach but now I want to solve it with the segment-tree based approach.

I have the consider the the values of a[i] as 0-based for my convinience in the segment tree.

The logic is to traverse from right to left in the given array, and put the contribution of a[i] in the segment tree as a point update.

I constructed a segment tree for the values of seniority, the root node of the current segment tree contains the minimum index for the seniority levels from 0 to k. The leaves contain the minimum index for the seniority levels 0,1,2,3,… k

The answer for each a[i], will be a range minimum query for the range 0 to a[i]-1 which will find the smallest index such that value at that index is smaller than a[i]

This code results into a WA.

But I am unable to find the issue in my thinking or code.

Can anyone help me out. Thank you in advance.

```
#include <bits/stdc++.h>
using namespace std;
#define fastio() \
ios_base::sync_with_stdio(false); \
cin.tie(NULL); \
cout.tie(NULL);
#define lli long long int
#define ll long long
#define pii pair<int,int>
ll INF=1e18+1000;
ll mod=1e9+7;
ll find(ll idx,ll low,ll high,ll l,ll r,ll seg[])
{
if(low>high) return INF;
if(r<low || high<l) return INF;
else if(l<=low && high<=r) return seg[idx];
ll mid=(low+high)>>1;
ll res_l=find(2*idx+1,low,mid,l,r,seg);
ll res_r=find(2*idx+2,mid+1,high,l,r,seg);
return min(res_l,res_r);
}
void update(ll idx,ll low,ll high,ll i,ll val,ll seg[])
{
if(low==high)
{
seg[idx]=val;
return;
}
ll mid=(low+high)>>1;
if(i<=mid) update(2*idx+1,low,mid,i,val,seg);
else update(2*idx+2,mid+1,high,i,val,seg);
seg[idx]=min(seg[2*idx+1],seg[2*idx+2]);
}
void solve()
{
ll n,k; cin>>n>>k;
ll a[n];
for(ll i=0;i<n;i++)
{ cin>>a[i]; a[i]--; }
ll M=4*k+10;
ll seg[M];
for(ll i=0;i<M;i++) seg[i]=INF;
ll ans=1;
for(ll i=n-1;i>=0;i--)
{
ll res=INF;
if(a[i]!=0) res=find(0,0,k-1,0,a[i]-1,seg);
update(0,0,k-1,a[i],i,seg);
if(res==INF) continue;
ans*=(res-i+1);
ans%=mod;
}
cout<<ans<<'\n';
return;
}
int main()
{
fastio();
int t=1;
// cin>>t;
while(t--) solve();
return 0;
}
```