算法描述：

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

Example:

Input: [1,null,2,3]
   1
    \
     2
    /
   3

Output: [1,2,3]

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

解题思路：先根非递归遍历。

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