E.cpp

#include<bits/stdc++.h>
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;

#define fo(i,n) for(i=0;i<n;i++)
#define fo2(i,start,end) for(i=start;i<=end;i++)
#define rfo(i,n) for(i=n-1;i>=0;i--)
#define deb(x) cerr<<#x<<"="<<x<<endl
#define deb2(x,y) cerr<<#x<<"="<<x<<","<<#y<<"="<<y<<endl
#define ll long long
#define pb push_back
#define mp make_pair
#define F first
#define S second
#define all(x) x.begin(), x.end()
#define sortall(x) sort(all(x))
#define tr(it,a) for(auto it=a.begin(); it!=a.end();it++)
#define nl '\n'
#define PI 3.1415926535897932384626
#define mod 1000000007

typedef pair<int, int>	pii;
typedef pair<ll, ll>	pl;
typedef vector<int>		vi;
typedef vector<ll>		vl;
typedef vector<pii>		vpii;
typedef vector<pl>		vpl;
typedef tree<ll, null_type, less<ll>, rb_tree_tag, tree_order_statistics_node_update > pbds; // find_by_order, order_of_key

const int N=1e6+10;
const ll inf = 1e9+10;
ll a[N],d;
bool helper(vl v,ll mid){
    ll i,last=0,mx=0,temp;
    // for(auto it : v){
    //     cerr<<it<<" ";
    // }cerr<<nl;
    fo(i,v.size()){
        if(v[i]-last-1<mid){
            return false;
        }
        mx = max(mx,v[i]-last-1);
        last = v[i];
    }

    temp = (mx-1)/2;
    temp = max(temp,d-v.back()-1);
    if(temp >= mid)return true;
    return false;

}
bool check(ll mid,int n){
    // deb(mid);
    int i,pos=-1,last=0;
    fo(i,n){
        if(a[i]-last-1<mid){
            pos=i;
            break;
        }
        last = a[i];
    }
    if(pos==-1)return true;

    vl v1,v2;
    fo(i,n){
        if(i!=pos){
            v1.pb(a[i]);
        }
        if(i!=pos-1){
            v2.pb(a[i]);
        }
    }

    if(helper(v1,mid) || helper(v2,mid)){
        return true;
    }
    return false;
}
void solve(){
    int i,j;
    ll n;
    cin>>n>>d;
    fo(i,n){
        cin>>a[i];
    }
    
    ll low,high,mid,ans=0;
    low = 0, high = inf;
    while(low<=high){
        mid = (low+high)/2;
        if(check(mid,n)){
            ans = mid;
            low = mid+1;
        }
        else{
            high = mid-1;
        }
    }
    cout<<ans<<nl;
}

int main(){
    // freopen("input.txt", "r", stdin);
    // freopen("output.txt", "w", stdout);
    ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
    int t=1;
    cin>>t;
    while(t--){
        solve();
    }
    
}