415-二叉树（144. 二叉树的前序遍历、145. 二叉树的后序遍历、94. 二叉树的中序遍历）

144. 二叉树的前序遍历

class Solution
{
    
public:
	void preorder(TreeNode* root, vector<int>& res)
	{
    
		if (root == nullptr)	return;

		res.push_back(root->val);
		preorder(root->left, res);
		preorder(root->right, res);
	}
public:
	vector<int> preorderTraversal(TreeNode* root)
	{
    
		vector<int> res;

		preorder(root, res);
		return res;
	}
};

class Solution
{
    
	vector<int> res;
public:
	void preorder(TreeNode* root)
	{
    
		if (root == nullptr)	return;

		res.push_back(root->val);
		preorder(root->left);
		preorder(root->right);
	}
public:
	vector<int> preorderTraversal(TreeNode* root)
	{
    
		preorder(root);
		return res;
	}
};

145. 二叉树的后序遍历

class Solution
{
    
	vector<int> res;
public:
	void preorder(TreeNode* root)
	{
    
		if (root == nullptr)	return;

		preorder(root->left);
		preorder(root->right);
		res.push_back(root->val);
	}
public:
	vector<int> postorderTraversal(TreeNode* root)
	{
    
		preorder(root);
		return res;
	}
};

94. 二叉树的中序遍历

class Solution 
{
    
	vector<int> res;
public:
	void preorder(TreeNode* root)
	{
    
		if (root == nullptr)	return;

		preorder(root->left);
		res.push_back(root->val);
		preorder(root->right);
	}
public:
	vector<int> inorderTraversal(TreeNode* root)
	{
    
		preorder(root);
		return res;
	}
};