CF1901A.Line Trip

There is a road, which can be represented as a number line. You are located in the point $0$ of the number line, and you want to travel from the point $0$ to the point $x$, and back to the point $0$.

You travel by car, which spends $1$ liter of gasoline per $1$ unit of distance travelled. When you start at the point $0$, your car is fully fueled (its gas tank contains the maximum possible amount of fuel).

There are $n$ gas stations, located in points $a_1, a_2, \dots, a_n$. When you arrive at a gas station, you fully refuel your car. Note that you can refuel only at gas stations, and there are no gas stations in points $0$ and $x$.

You have to calculate the minimum possible volume of the gas tank in your car (in liters) that will allow you to travel from the point $0$ to the point $x$ and back to the point $0$.

有一条道路，可以表示为一条数轴。你位于数轴上的点 $0$，需要从点 $0$ 出发到达点 $x$，再返回点 $0$。

你驾驶一辆汽车，每行驶 $1$ 单位距离消耗 $1$ 升汽油。当你从点 $0$ 出发时，汽车油箱已加满（即装有最大可能的油量）。

共有 $n$ 个加油站，分别位于点 $a_1, a_2, \dots, a_n$。当你抵达一个加油站时，你的汽车会被完全加满。注意：你只能在加油站加油，且点 $0$ 和点 $x$ 处均无加油站。

你需要计算能使你顺利完成从点 $0$ 到点 $x$ 再返回点 $0$ 的行程所需的汽车油箱最小容积（单位：升）。

The first line contains one integer $t$ ($1 \le t \le 1000$) — the number of test cases.

Each test case consists of two lines:

• the first line contains two integers $n$ and $x$ ($1 \le n \le 50$; $2 \le x \le 100$);
• the second line contains $n$ integers $a_1, a_2, \dots, a_n$ ($0 \lt a_1 \lt a_2 \lt \dots \lt a_n \lt x$).

第一行包含一个整数 $t$（$1 \le t \le 1000$）—— 表示测试用例的数量。

每个测试用例由两行组成：

• 第一行包含两个整数 $n$ 和 $x$（$1 \le n \le 50$；$2 \le x \le 100$）；
• 第二行包含 $n$ 个整数 $a_1, a_2, \dots, a_n$（$0 \lt a_1 \lt a_2 \lt \dots \lt a_n \lt x$）。

For each test case, print one integer — the minimum possible volume of the gas tank in your car that will allow you to travel from the point $0$ to the point $x$ and back.

对于每个测试用例，输出一个整数——使汽车能够从位置 $0$ 行驶到位置 $x$ 并返回所需的最小油箱容积。

In the first test case of the example, if the car has a gas tank of $4$ liters, you can travel to $x$ and back as follows:

• travel to the point $1$, then your car's gas tank contains $3$ liters of fuel;
• refuel at the point $1$, then your car's gas tank contains $4$ liters of fuel;
• travel to the point $2$, then your car's gas tank contains $3$ liters of fuel;
• refuel at the point $2$, then your car's gas tank contains $4$ liters of fuel;
• travel to the point $5$, then your car's gas tank contains $1$ liter of fuel;
• refuel at the point $5$, then your car's gas tank contains $4$ liters of fuel;
• travel to the point $7$, then your car's gas tank contains $2$ liters of fuel;
• travel to the point $5$, then your car's gas tank contains $0$ liters of fuel;
• refuel at the point $5$, then your car's gas tank contains $4$ liters of fuel;
• travel to the point $2$, then your car's gas tank contains $1$ liter of fuel;
• refuel at the point $2$, then your car's gas tank contains $4$ liters of fuel;
• travel to the point $1$, then your car's gas tank contains $3$ liters of fuel;
• refuel at the point $1$, then your car's gas tank contains $4$ liters of fuel;
• travel to the point $0$, then your car's gas tank contains $3$ liters of fuel.

在示例的第一个测试用例中，若汽车的油箱容量为 $4$ 升，则可以按如下方式抵达位置 $x$ 并返回：

• 行驶至位置 $1$，此时汽车油箱剩余 $3$ 升燃油；
• 在位置 $1$ 加油，此时汽车油箱变为 $4$ 升燃油；
• 行驶至位置 $2$，此时汽车油箱剩余 $3$ 升燃油；
• 在位置 $2$ 加油，此时汽车油箱变为 $4$ 升燃油；
• 行驶至位置 $5$，此时汽车油箱剩余 $1$ 升燃油；
• 在位置 $5$ 加油，此时汽车油箱变为 $4$ 升燃油；
• 行驶至位置 $7$，此时汽车油箱剩余 $2$ 升燃油；
• 行驶至位置 $5$，此时汽车油箱剩余 $0$ 升燃油；
• 在位置 $5$ 加油，此时汽车油箱变为 $4$ 升燃油；
• 行驶至位置 $2$，此时汽车油箱剩余 $1$ 升燃油；
• 在位置 $2$ 加油，此时汽车油箱变为 $4$ 升燃油；
• 行驶至位置 $1$，此时汽车油箱剩余 $3$ 升燃油；
• 在位置 $1$ 加油，此时汽车油箱变为 $4$ 升燃油；
• 行驶至位置 $0$，此时汽车油箱剩余 $3$ 升燃油。