LeetCode OJ - 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].

Note: Recursive solution is trivial, could you do it iteratively?

递归代码

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