#include <bits/stdc++.h>

using namespace std;

int main() {
    int t;
    cin >> t;
    while (t--) {
        int n;
        cin >> n;
        vector<int> nums(n);
        for (int i = 0; i < n; i++)
            cin >> nums[i];

        sort(nums.begin(), nums.end());

        int minDiff = numeric_limits<int>::min();
        for (int i = 1; i < n; i+=2) {
            minDiff = max(minDiff, nums[i] - nums[i - 1]);
        }

        cout << minDiff << endl;
    }
    return 0;
}