• 首先实现一颗这样的树
  
  

• 然后使用系统栈(递归)和自己定义的栈分别实现二叉树的前序遍历

c++代码

#include<iostream>
#include<stack>
#include<map>

using namespace std;

map<int, char> nodeMap;


struct TreeNode
{
	int val_;
	TreeNode* left_;
	TreeNode* right_;

	TreeNode(const int val, TreeNode* left, TreeNode* right) :val_(val), left_(left), right_(right) {}
	TreeNode() :TreeNode(0, nullptr, nullptr) {}
	TreeNode(const int val) :TreeNode(val, nullptr, nullptr) {}
};

void preorderTraversal(TreeNode* root)
{
	if (root == nullptr)
	{		
		return;
	}
	cout << nodeMap[root->val_] << " ";
	preorderTraversal(root->left_);
	preorderTraversal(root->right_);
}

void preorderTraversal_customStack(TreeNode* root)
{
	stack<TreeNode*> customStack;
	customStack.push(root);
	while (customStack.empty() == false)
	{
		// 访问根节点
		TreeNode* tmpnode = customStack.top();
		customStack.pop();
		cout << nodeMap[tmpnode->val_] << " ";
		if (tmpnode->right_ != nullptr) 
		{
			// 右节点入栈
			customStack.push(tmpnode->right_);
		}
		if (tmpnode->left_ != nullptr)
		{
			// 左节点入栈
			customStack.push(tmpnode->left_);
		}
	}
	cout << endl;
}

int main()
{
	nodeMap[11] = 'A';
	nodeMap[9] = 'B';
	nodeMap[7] = 'C';
	nodeMap[3] = 'D';
	nodeMap[5] = 'E';
	nodeMap[6] = 'F';
	nodeMap[1] = 'G';
	nodeMap[2] = 'H';
	nodeMap[8] = 'I';
	nodeMap[4] = 'J';
	nodeMap[10] = 'K';

	TreeNode* root = new TreeNode(11);

	TreeNode* node1 = new TreeNode(9);
	root->left_ = node1;

	TreeNode* node2 = new TreeNode(7);	
	root->right_ = node2;

	TreeNode* node3 = new TreeNode(3);	
	node1->left_ = node3;

	TreeNode* node4 = new TreeNode(5);	
	node1->right_ = node4;

	TreeNode* node5 = new TreeNode(6);	
	node2->left_ = node5;

	TreeNode* node6 = new TreeNode(1);
	node2->right_ = node6;

	TreeNode* node7 = new TreeNode(2);
	node3->left_ = node7;

	TreeNode* node8 = new TreeNode(8);
	node3->right_ = node8;

	TreeNode* node9 = new TreeNode(4);
	node5->right_ = node9;

	TreeNode* node10 = new TreeNode(10);
	node6->left_ = node10;

	preorderTraversal(root);
	cout << endl;

	preorderTraversal_customStack(root);

	return 0;
}

结果

与预期结果是一致的