06 - Morris Preorder Traversal.cpp

/*
MORRIS PREORDER TRAVERSAL
*/

/**
 * 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> preorder;
    if(root == NULL)
    return preorder;

    TreeNode* curr = root;
    while(curr!=NULL)
    {
        if(curr->left == NULL)
        {
             preorder.push_back(curr -> val);
             curr = curr -> right;
        }
        else
        {
            TreeNode* prev = curr->left;
            while(prev -> right!=NULL && prev ->right!=curr)
                prev = prev -> right;
            
            if(prev -> right == NULL)
            {
                prev -> right = curr;
                preorder.push_back(curr -> val);
                curr = curr -> left;
            }
            else
            {
                prev -> right = NULL;
                curr = curr -> right;
            }
        }
    }
        return preorder;
        
    }
};