Binary Tree Preorder Traversal

 

 先序遍历，就是时间复杂度有点高

class Solution {
public:
    vector<int>a;
    vector<int> preorderTraversal(TreeNode* root)
    {
        if(root)
        {
            a.push_back(root->val);
            preorderTraversal(root->left);
            preorderTraversal(root->right);
        }
        return a;
    }
};