层次遍历二叉树

《剑指Offer——名企面试官精讲典型编程题》代码

  著作权所有者：何海涛

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;
}