《剑指offer》第三十二题III：之字形打印二叉树

// 面试题32（三）：之字形打印二叉树
// 题目：请实现一个函数按照之字形顺序打印二叉树，即第一行按照从左到右的顺
// 序打印，第二层按照从右到左的顺序打印，第三行再按照从左到右的顺序打印，
// 其他行以此类推。

#include <cstdio>
#include "BinaryTree.h"
#include <stack>

void Print(BinaryTreeNode* pRoot)
{
    //整体思路和前两道题很像,先进先出变后进先出

    if (pRoot == nullptr)
        return;

    std::stack<BinaryTreeNode*> levels[2]; //创建两个栈
    int current = 0; //当前为奇数, 从左到右
    int next = 1; //下一层为偶数, 从右向左

    levels[current].push(pRoot);

    while (!levels[current].empty() || !levels[next].empty())
    {
        BinaryTreeNode* pNode = levels[current].top();
        printf("%d ", pNode->m_nValue);
        levels[current].pop();

        if (current == 0) //当前打印奇数序列, 从左到右
        {
            if (pNode->m_pLeft)
                levels[next].push(pNode->m_pLeft);
            if (pNode->m_pRight)
                levels[next].push(pNode->m_pRight);
        }
        else //当前打印偶数序列, 从右到左
        {
            if (pNode->m_pRight)
                levels[next].push(pNode->m_pRight);
            if (pNode->m_pLeft)
                levels[next].push(pNode->m_pLeft);
        }

        if (levels[current].empty()) //如果当前节点打印完了
        {
            printf("\n");
            current = 1 - current;
            next = 1 - next;
        }
    }
}

// ====================测试代码====================
//            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, nullptr);
    ConnectTreeNodes(pNode4, pNode3, nullptr);
    ConnectTreeNodes(pNode3, pNode2, nullptr);

    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, nullptr, pNode4);
    ConnectTreeNodes(pNode4, nullptr, pNode3);
    ConnectTreeNodes(pNode3, nullptr, 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(nullptr);
    printf("\n");
}

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

    ConnectTreeNodes(pNode100, pNode50, nullptr);
    ConnectTreeNodes(pNode50, nullptr, 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;
}

分析：举例分析，奇偶数行打印顺序不同，所以下一行压入栈顺序也不同。

/*
struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
    TreeNode(int x) :
            val(x), left(NULL), right(NULL) {
    }
};
*/
class Solution {
public:
    vector<vector<int> > Print(TreeNode* pRoot) {
        
        std::stack<TreeNode*> levels[2];
        int current = 0;
        int next = 1;
        
        std::vector<vector<int> > printTreeNode;
        std::vector<int> printTemp;
        
        if (pRoot == nullptr)
            return printTreeNode;
        
        levels[current].push(pRoot);
        while (!levels[current].empty() || !levels[next].empty())
        {
            TreeNode* pNode = levels[current].top();
            levels[current].pop();
            
            printTemp.push_back(pNode->val);
            
            if (current == 0)
            {
                if (pNode->left)
                    levels[next].push(pNode->left);
                if (pNode->right)
                    levels[next].push(pNode->right);
            }
            else
            {
                if (pNode->right)
                    levels[next].push(pNode->right);
                if (pNode->left)
                    levels[next].push(pNode->left);
            }
            
            if (levels[current].empty())
            {
                printTreeNode.push_back(printTemp);
                printTemp.clear();
                current = 1 - current;
                next = 1 - next;
            }
        }
        return printTreeNode;
    }
    
};