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/**
 * 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> res;
        stack<TreeNode*> stk;
        while(root || stk.size()) {
            while(root) {
                // 遍历当前节点
                res.push_back(root->val);
                stk.push(root);
                // 遍历左子树
                root = root->left;
            }
            // 从栈中取出后继
            root = stk.top()->right;
            stk.pop();
        }
        return res;
    }
};