Binary Tree Preorder Traversal

/**
 * Definition for binary tree
 * 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> ret;
        
        // 1
        /*
        stack<TreeNode*> tsk;
        TreeNode *cur = root;
        auto it = back_inserter(ret);
        while (!tsk.empty() || cur != NULL) {
            if (cur != NULL) {
                *it = cur->val;
                tsk.push(cur);
                cur = cur->left;
                continue;
            }
            
            cur = tsk.top()->right;
            tsk.pop();
        }
        */
        
        // 2
        TreeNode *cur = root;
        auto it = back_inserter(ret);
        while (cur != NULL) {
            if (cur->left == NULL) {
                *it = cur->val;
                cur = cur->right;
                continue;
            }
            
            TreeNode *ll = cur->left;
            while (ll->right != NULL && ll->right != cur) ll = ll->right;
            if (ll->right == cur) {
                cur = cur->right;
                ll->right = NULL;
            } else {
                *it = cur->val;
                ll->right = cur;
                cur = cur->left;
            }
        }
        
        return ret;
    }
};