按照之字形顺序打印二叉树

问题：

请实现一个函数按照之字形打印二叉树，即第一行按照从左到右的顺序打印，第二层按照从右至左的顺序打印，第三行按照从左到右的顺序打印，其他行以此类推。例如:

分析：

按照广度优先遍历来遍历二叉树，但是需要按照之字形来打印：
奇数行从左到右，跟BFS的遍历顺序一样，而偶数行从右到左，跟BFS的遍历顺序相反。
因此可以通过两个栈来实现，一个实现先进先出，即入栈顺序为右子节点、左子节点；一个实现后进先出，即入栈顺序为左子节点、右子节点
详细的实现代码如下:
//将树打印之字形
void Print(BinaryTreeNode* pRoot)
{
if(pRoot == NULL)
return;

std::stack<BinaryTreeNode*> levels[2];
int current = 0;
int next = 1;

levels[current].push(pRoot);
while(!levels[0].empty() || !levels[1].empty())
{
BinaryTreeNode* pNode = levels[current].top();
levels[current].pop();

printf("%d ", pNode->m_nValue);

if(current == 0)
{
if(pNode->m_pLeft != NULL)
levels[next].push(pNode->m_pLeft);
if(pNode->m_pRight != NULL)
levels[next].push(pNode->m_pRight);
}
else
{
if(pNode->m_pRight != NULL)
levels[next].push(pNode->m_pRight);
if(pNode->m_pLeft != NULL)
levels[next].push(pNode->m_pLeft);
}

if(levels[current].empty())
{
printf("\n");
current = 1 - current;
next = 1 - next;
}
}
}

// 8
// 6 10
// 5 7 9 11
void Test1()
{
BinaryTreeNode* pNode8 = CreateBinaryTreeNode(8);
BinaryTreeNode* pNode6 = CreateBinaryTreeNode(6);
BinaryTreeNode* pNode10 = CreateBinaryTreeNode(10);
BinaryTreeNode* pNode5 = CreateBinaryTreeNode(5);
BinaryTreeNode* pNode7 = CreateBinaryTreeNode(7);
BinaryTreeNode* pNode9 = CreateBinaryTreeNode(9);
BinaryTreeNode* pNode11 = CreateBinaryTreeNode(11);

ConnectTreeNodes(pNode8, pNode6, pNode10);
ConnectTreeNodes(pNode6, pNode5, pNode7);
ConnectTreeNodes(pNode10, pNode9, pNode11);

printf("====Test1 Begins: ====\n");
printf("Expected Result is:\n");
printf("8 \n");
printf("10 6 \n");
printf("5 7 9 11 \n\n");

printf("Actual Result is: \n");
Print(pNode8);
printf("\n");

DestroyTree(pNode8);
}

// 5
// 4
// 3
// 2
void Test2()
{
BinaryTreeNode* pNode5 = CreateBinaryTreeNode(5);
BinaryTreeNode* pNode4 = CreateBinaryTreeNode(4);
BinaryTreeNode* pNode3 = CreateBinaryTreeNode(3);
BinaryTreeNode* pNode2 = CreateBinaryTreeNode(2);

ConnectTreeNodes(pNode5, pNode4, NULL);
ConnectTreeNodes(pNode4, pNode3, NULL);
ConnectTreeNodes(pNode3, pNode2, NULL);

printf("====Test2 Begins: ====\n");
printf("Expected Result is:\n");
printf("5 \n");
printf("4 \n");
printf("3 \n");
printf("2 \n\n");

printf("Actual Result is: \n");
Print(pNode5);
printf("\n");

DestroyTree(pNode5);
}

// 5
// 4
// 3
// 2
void Test3()
{
BinaryTreeNode* pNode5 = CreateBinaryTreeNode(5);
BinaryTreeNode* pNode4 = CreateBinaryTreeNode(4);
BinaryTreeNode* pNode3 = CreateBinaryTreeNode(3);
BinaryTreeNode* pNode2 = CreateBinaryTreeNode(2);

ConnectTreeNodes(pNode5, NULL, pNode4);
ConnectTreeNodes(pNode4, NULL, pNode3);
ConnectTreeNodes(pNode3, NULL, pNode2);

printf("====Test3 Begins: ====\n");
printf("Expected Result is:\n");
printf("5 \n");
printf("4 \n");
printf("3 \n");
printf("2 \n\n");

printf("Actual Result is: \n");
Print(pNode5);
printf("\n");

DestroyTree(pNode5);
}

void Test4()
{
BinaryTreeNode* pNode5 = CreateBinaryTreeNode(5);

printf("====Test4 Begins: ====\n");
printf("Expected Result is:\n");
printf("5 \n\n");

printf("Actual Result is: \n");
Print(pNode5);
printf("\n");

DestroyTree(pNode5);
}

void Test5()
{
printf("====Test5 Begins: ====\n");
printf("Expected Result is:\n");

printf("Actual Result is: \n");
Print(NULL);
printf("\n");
}

// 100
// /
// 50
// \
// 150
void Test6()
{
BinaryTreeNode* pNode100 = CreateBinaryTreeNode(100);
BinaryTreeNode* pNode50 = CreateBinaryTreeNode(50);
BinaryTreeNode* pNode150 = CreateBinaryTreeNode(150);

ConnectTreeNodes(pNode100, pNode50, NULL);
ConnectTreeNodes(pNode50, NULL, pNode150);

printf("====Test6 Begins: ====\n");
printf("Expected Result is:\n");
printf("100 \n");
printf("50 \n");
printf("150 \n\n");

printf("Actual Result is: \n");
Print(pNode100);
printf("\n");
}

// 8
// 4 12
// 2 6 10 14
// 1 3 5 7 9 11 13 15
void Test7()
{
BinaryTreeNode* pNode8 = CreateBinaryTreeNode(8);
BinaryTreeNode* pNode4 = CreateBinaryTreeNode(4);
BinaryTreeNode* pNode12 = CreateBinaryTreeNode(12);
BinaryTreeNode* pNode2 = CreateBinaryTreeNode(2);
BinaryTreeNode* pNode6 = CreateBinaryTreeNode(6);
BinaryTreeNode* pNode10 = CreateBinaryTreeNode(10);
BinaryTreeNode* pNode14 = CreateBinaryTreeNode(14);
BinaryTreeNode* pNode1 = CreateBinaryTreeNode(1);
BinaryTreeNode* pNode3 = CreateBinaryTreeNode(3);
BinaryTreeNode* pNode5 = CreateBinaryTreeNode(5);
BinaryTreeNode* pNode7 = CreateBinaryTreeNode(7);
BinaryTreeNode* pNode9 = CreateBinaryTreeNode(9);
BinaryTreeNode* pNode11 = CreateBinaryTreeNode(11);
BinaryTreeNode* pNode13 = CreateBinaryTreeNode(13);
BinaryTreeNode* pNode15 = CreateBinaryTreeNode(15);

ConnectTreeNodes(pNode8, pNode4, pNode12);
ConnectTreeNodes(pNode4, pNode2, pNode6);
ConnectTreeNodes(pNode12, pNode10, pNode14);
ConnectTreeNodes(pNode2, pNode1, pNode3);
ConnectTreeNodes(pNode6, pNode5, pNode7);
ConnectTreeNodes(pNode10, pNode9, pNode11);
ConnectTreeNodes(pNode14, pNode13, pNode15);

printf("====Test7 Begins: ====\n");
printf("Expected Result is:\n");
printf("8 \n");
printf("12 4 \n");
printf("2 6 10 14 \n");
printf("15 13 11 9 7 5 3 1 \n\n");

printf("Actual Result is: \n");
Print(pNode8);
printf("\n");

DestroyTree(pNode8);
}

int main(int argc, char* argv[])
{
Test1();
Test2();
Test3();
Test4();
Test5();
Test6();
Test7();

return 0;
}