Leetcode144.Binary_Tree_Preorder_Traversal

前序遍历的递归写法是显然的，这里只给出非递归的解法（用栈实现）。
时间复杂度:O(N)
C++代码：

class Solution {
public:
	vector<int> preorderTraversal(TreeNode* root) {
		if (root == nullptr)
			return {};
		vector<int> result;
		stack<TreeNode*> record;
		TreeNode* temp = root;
		while (temp != nullptr || !record.empty())
		{
			while (temp != nullptr)
			{
				result.push_back(temp->val);
				record.push(temp);
				temp = temp->left;
			}
			if (!record.empty())
			{
				temp = record.top();
				record.pop();
				temp = temp->right;
			}
		}
		return result;
	}
};