BT_Preorder_Morris.cpp

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> res;
        TreeNode* node = root;
        while(node != NULL){
            if(node->left == NULL){
                res.push_back(node->val);
                node = node->right;
            }
            else{
                TreeNode* predecessor = node->left;
                while(predecessor->right && predecessor->right != node){
                    predecessor = predecessor->right;
                }
                if(predecessor->right == NULL){
                    res.push_back(node->val);
                    predecessor->right = node;
                    node = node->left;
                }
                else{
                    node = node->right;
                    predecessor->right = NULL;
                }
            }
        }
        return res;
    }
};