morris traversal preorder

/**
 * 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> res;
    vector<int> preorderTraversal(TreeNode* root) {
        // if(root==nullptr) return res;
        // res.push_back(root->val);
        // preorderTraversal(root->left);
        // preorderTraversal(root->right);
        // return res;
        TreeNode* curr = root;
        while(curr){
            res.push_back(curr->val);
            if(!curr->left) curr= curr->right;
            else{
                TreeNode* temp = curr->left;
                while(temp->right) temp=temp->right;
                temp->right = curr->right;
                curr->right = NULL;
                curr = curr->left;
            }
        }
        return res;
    }
};