144. Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes’ values.

For example:
Given binary tree {1,#,2,3},
1
\
2
/
3
return [1,2,3].

/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    vector<int> result;    vector<int> preorderTraversal(TreeNode* root) {        if(root==NULL) return result;        preorder(root);        return result;    }    void preorder(TreeNode* root){        if(root==NULL) return ;        result.push_back(root->val);        preorder(root->left);        preorder(root->right);        return;    }};