剑指Offer面试题：20.栈的压入、弹出序列

    public static bool IsPopOrder(int[] pushOrder, int[] popOrder, int length)
    {
        bool possible = false;

        if (pushOrder != null && popOrder != null && length > 0)
        {
            int nextPush = 0; // 指向下一个要push的元素的index
            int nextPop = 0;  // 指向下一个要pop的元素的index
            int pop = 0;      // 指向popOrder的首个元素的index
            int push = 0;     // 指向pushOrder的首个元素的index

            Stack<int> stackData = new Stack<int>();
            while (nextPop - pop < length)
            {
                // 当辅助栈的栈顶元素不是要弹出的元素
                // 先压入一些数字入栈
                while (stackData.Count == 0 || stackData.Peek() != popOrder[nextPop])
                {
                    // 如果所有数字都压入辅助栈了，退出循环
                    if (nextPush - push == length)
                    {
                        break;
                    }

                    stackData.Push(pushOrder[nextPush]);
                    nextPush++;
                }

                // 说明没有匹配成功
                if (stackData.Peek() != popOrder[nextPop])
                {
                    break;
                }

                stackData.Pop();
                nextPop++;
            }

            if (stackData.Count == 0 && nextPop - pop == length)
            {
                possible = true;
            }
        }

        return possible;
    }

    [TestMethod]
    public void OrderTest1()
    {
        int length = 5;
        int[] push = { 1, 2, 3, 4, 5 };
        int[] pop = { 4, 5, 3, 2, 1 };

        Assert.AreEqual(StackHelper.IsPopOrder(push, pop, length), true);
    }

    [TestMethod]
    public void OrderTest2()
    {
        int length = 5;
        int[] push = { 1, 2, 3, 4, 5 };
        int[] pop = { 3, 5, 4, 2, 1 };

        Assert.AreEqual(StackHelper.IsPopOrder(push, pop, length), true);
    }

    [TestMethod]
    public void OrderTest3()
    {
        int length = 5;
        int[] push = { 1, 2, 3, 4, 5 };
        int[] pop = { 4, 3, 5, 1, 2 };

        Assert.AreEqual(StackHelper.IsPopOrder(push, pop, length), false);
    }

    [TestMethod]
    public void OrderTest4()
    {
        int length = 5;
        int[] push = { 1, 2, 3, 4, 5 };
        int[] pop = { 3, 5, 4, 1, 2 };

        Assert.AreEqual(StackHelper.IsPopOrder(push, pop, length), false);
    }

    // push和pop序列只有一个数字且不同
    [TestMethod]
    public void OrderTest5()
    {
        int length = 1;
        int[] push = { 1 };
        int[] pop = { 2 };

        Assert.AreEqual(StackHelper.IsPopOrder(push, pop, length), false);
    }

    // push和pop序列只有一个数字且相同
    [TestMethod]
    public void OrderTest6()
    {
        int length = 1;
        int[] push = { 1 };
        int[] pop = { 1 };

        Assert.AreEqual(StackHelper.IsPopOrder(push, pop, length), true);
    }

    // NULL指针
    [TestMethod]
    public void OrderTest7()
    {
        Assert.AreEqual(StackHelper.IsPopOrder(null, null, 0), false);
    }