层次遍历二叉树

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著作权所有者：何海涛
《剑指Offer——名企面试官精讲典型编程题》代码

题目23: 层次遍历二叉树
题目: 从上到下打印二叉树的每个结点，同一层的结点按照从左到右的顺序打印。

#include <iostream>#include <exception>#include <queue>using namespace std;struct BinaryTreeNode{    int                    m_nValue;    BinaryTreeNode*        m_pLeft;    BinaryTreeNode*        m_pRight;};BinaryTreeNode* ConstructCore(int* startPreorder, int* endPreorder, int* startInorder, int* endInorder);BinaryTreeNode* Construct(int* preorder, int* inorder, int length){    if (preorder == NULL || inorder == NULL || length <= 0)        return NULL;    return ConstructCore(preorder, preorder + length - 1,        inorder, inorder + length - 1);}BinaryTreeNode* ConstructCore(int* startPreorder, int* endPreorder,int* startInorder, int* endInorder){    // 前序遍历序列的第一个数字是根结点的值    int rootValue = startPreorder[0];    BinaryTreeNode* root = new BinaryTreeNode();    root->m_nValue = rootValue;    root->m_pLeft = root->m_pRight = NULL;    if (startPreorder == endPreorder)    {        if (startInorder == endInorder && *startPreorder == *startInorder)            return root;        else            throw std::exception("Invalid input.");    }    // 在中序遍历中找到根结点的值    int* rootInorder = startInorder;    while (rootInorder <= endInorder && *rootInorder != rootValue)        ++rootInorder;    if (rootInorder == endInorder && *rootInorder != rootValue)        throw std::exception("Invalid input.");    int leftLength = rootInorder - startInorder;    int* leftPreorderEnd = startPreorder + leftLength;    if (leftLength > 0)    {        // 构建左子树        root->m_pLeft = ConstructCore(startPreorder + 1, leftPreorderEnd,            startInorder, rootInorder - 1);    }    if (leftLength < endPreorder - startPreorder)    {        // 构建右子树        root->m_pRight = ConstructCore(leftPreorderEnd + 1, endPreorder,            rootInorder + 1, endInorder);    }    return root;}void PrintFromTopToBottom(BinaryTreeNode* rRoot){    if (rRoot == NULL)        return;    queue<BinaryTreeNode*> queueNode;    queueNode.push(rRoot);    while (!queueNode.empty())    {        BinaryTreeNode * pNode = queueNode.front();        queueNode.pop();        cout << pNode->m_nValue << " ";        if (pNode->m_pLeft != nullptr)            queueNode.push(pNode->m_pLeft);        if (pNode->m_pRight != nullptr)            queueNode.push(pNode->m_pRight);    }}int main(){    const int length = 8;    int preorder[length] = { 1, 2, 4, 7, 3, 5, 6, 8 };    int inorder[length] = { 4, 7, 2, 1, 5, 3, 8, 6 };    BinaryTreeNode* root = nullptr;    try    {        root = Construct(preorder, inorder, length);    }    catch (std::exception& exception)    {        printf("Invalid Input.\n");    }    PrintFromTopToBottom(root);    return 0;}

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