gistfile1.cs

// The code below would print overlapped A and B sequences like:
// ...
// A
// A
// B
// B
// ...
//
// Please add multi-threading protection code to make sure that
// As and Bs are printed in the order of:
// ...
// A
// B
// A
// B
// ...

static object LOCK = new object();
static bool A = true;

static void PrintA()
{
lock (LOCK)
            {
                if (A)
                {
                    Console.WriteLine("A");
                    A = false;
                }
            }
}

static void PrintB()
{
    lock (LOCK)
            {
                if (!A)
                {
                    Console.WriteLine("B");
                    A = true;
                }
            }
}

// DO NOT touch the code below

for (var i = 0; i < 10; i++)
{
    new Thread(() => {
        while (true) {
            PrintA();
            PrintB();
        }
    }).Start();
}

要求很简单 就不上复杂的同步手段了：）