import io, os
input = io.BytesIO(os.read(0, os.fstat(0).st_size)).readline
t = int(input())
for tidx in range(t):
input()
n, d = [int(x) for x in input().split()]
a = [int(x) for x in input().split()]
b = [a[i+1]-a[i]-1 for i in range(n-1)]
b.append(a[0]-1)
mu = min(b)
maxb = max((max(b)-1)//2, d-max(a)-1)
idx = b.index(mu) if idx == n-1:
b[n-1] = d
b[0] = a[1]-1
maxb = max(maxb, (a[1]-2)//2)
print(max(min(min(b), maxb), mu))
else:
b2 = b[:]
if idx>0:
c = a[idx+1]-a[idx-1]-1
b[idx-1] = c
b[idx] = d
else:
c = a[1]-1
b[n-1] = c
b[idx] = d
m = min(min(b), max(maxb, (c-1)//2))
b = b2
if idx<n-2:
c = a[idx+2]-a[idx]-1
b[idx] = c
b[idx+1] = d
m = max(m, min(min(b), max(maxb, (c-1)//2)))
else:
b[n-2] = d-a[n-2]-1
m = max(m, min(b))
print(max(m, mu))
#include <bits/stdc++.h>
#define int long long
#define mp make_pair
#define x first
#define y second
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
typedef long double ld;
typedef long long ll;
using namespace std;
mt19937 rnd(143);
const ll inf = 1e9;
const ll M = 998'244'353;
const ld pi = atan2(0, -1);
const ld eps = 1e-4;
int n, d;
int cnt(vector<int> &schedule){
int mx = 0, mn = inf;
for(int i = 1; i < n; ++i){
mx = max(mx, schedule[i] - schedule[i - 1] - 1);
mn = min(mn, schedule[i] - schedule[i - 1] - 1);
}
return min(mn, max(d - schedule.back() - 1, (mx - 1) / 2));
}
void solve(int test_case) {
cin >> n >> d;
vector<int> a(n + 1);
int mn = d, min_pos = 0;
for(int i = 1; i <= n; ++i){
cin >> a[i];
if(a[i] - a[i - 1] - 1 < mn){
mn = a[i] - a[i - 1] - 1;
min_pos = i;
}
}
vector<int> schedule;
for(int i = 0; i <= n; ++i){
if(i != min_pos){
schedule.push_back(a[i]);
}
}
int ans = cnt(schedule);
if(min_pos > 1){
schedule[min_pos - 1] = a[min_pos];
}
ans = max(ans, cnt(schedule));
cout << ans;
}
bool multi = true;
signed main() {
//freopen("in.txt", "r", stdin);
//freopen("in.txt", "w", stdout);
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cout.tie(nullptr);
int t = 1;
if (multi) {
cin >> t;
}
for (int i = 1; i <= t; ++i) {
solve(i);
cout << "\n";
}
return 0;
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