Solution2.cpp

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> ans;
        if (!root) {
            return ans;
        }
        stack<TreeNode*> stk;
        stk.push(root);
        while (stk.size()) {
            auto node = stk.top();
            stk.pop();
            ans.push_back(node->val);
            if (node->right) {
                stk.push(node->right);
            }
            if (node->left) {
                stk.push(node->left);
            }
        }
        return ans;
    }
};