5.3 二叉树的遍历和线索二叉树

void preorder(TreeNode *root, vector<int> &res)
    {
        if (!root)
        {
            return;
        }
        res.push_back(root->val);
        preorder(root->left, res);
        preorder(root->right, res);
    }
    vector<int> preorderTraversal(TreeNode *root)
    {
        vector<int> res;
        preorder(root, res);
        return res;
    }

void inorder(TreeNode *root, vector<int> &res)
    {
        if (!root)
        {
            return;
        }
        inorder(root->left, res);
        res.push_back(root->val);
        inorder(root->right, res);
    };
    vector<int> inorderTraversal(TreeNode *root)
    {
        vector<int> res;
        inorder(root, res);
        return res;
    }

void postorder(TreeNode *root, vector<int> &res)
    {
        if (!root)
        {
            return;
        }
        postorder(root->left, res);
        postorder(root->right, res);
        res.push_back(root->val);
    }
    vector<int> postorderTraversal(TreeNode *root)
    {
        vector<int> res;
        postorder(root, res);
        return res;
    }