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; }