面试题32（二）：分行从上到下打印二叉树

一、题目

从上到下按层打印二叉树，同一层的结点按从左到右的顺序打印，每一层打印到一行。

二、关键

1.队列

2.两个变量。

三、解释

1.队列：使用队列保存将要打印的节点。

2.两个变量：把二叉树的每一行单独打印到一行中。一个变量表示当前层中还没有打印的节点数；另个一变量表示下一层节点的数目。

3.思路：变量toBePrinted表示在当前层中还没有打印的节点数，变量nextLevel表示下一层的节点数。如果一个节点有子节点，则把每一个子节点加入队列，同时把变量nextLevel加1.每打印一个节点，toBePrinted减1.当toBePrinted变成0时，表示当前层的所有节点已经打印完毕，可以继续打印下一层。

四、代码

#include <cstdio>
#include "..\Utilities\BinaryTree.h"
#include <queue>

void Print(BinaryTreeNode* pRoot)
{
    if(pRoot == nullptr)
        return;

    std::queue<BinaryTreeNode*> nodes;
    nodes.push(pRoot);
    int nextLevel = 0;
    int toBePrinted = 1;
    while(!nodes.empty())
    {
        BinaryTreeNode* pNode = nodes.front();
        printf("%d ", pNode->m_nValue);

        if(pNode->m_pLeft != nullptr)
        {
            nodes.push(pNode->m_pLeft);
            ++nextLevel;
        }
        if(pNode->m_pRight != nullptr)
        {
            nodes.push(pNode->m_pRight);
            ++nextLevel;
        }

        nodes.pop();
        --toBePrinted;
        if(toBePrinted == 0)
        {
            printf("\n");
            toBePrinted = nextLevel;
            nextLevel = 0;
        }
    }
}

// ====================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("6 10 \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");
}

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

    return 0;
}