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;

}
};