給定一個二叉樹，返回它的 前序 遍歷。

示例:

輸入: [1,null,2,3]
1

2
/
3

輸出: [1,2,3]
進階: 遞迴演算法很簡單，你可以通過迭代演算法完成嗎？

/**
 * 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> preorderTraversal(TreeNode* root) {
		vector<int> result;
		stack<TreeNode*> sta;
		//result.push_back(root->val);
		TreeNode* node = root;
		while (node != NULL || !sta.empty())
		{
			while (node != NULL) 
			{
				sta.push(node);
				result.push_back(node->val)
 
;
				node = node->left;
			}

			if (!sta.empty())
			{
				node = sta.top();
				sta.pop();
				node = node->right;
			}
		}
		return result;
    }
};