Preorder_Traversal_Iterative.

class Solution {
public:
	vector<int> result;
	vector<int> preorderTraversal(TreeNode* root) {
	    if(!root)
	        return result;
		set<TreeNode*> explored;
		vector<TreeNode*> res;
		res.push_back(root);
		result.push_back(root->val);
		TreeNode* node = root;
		while (res.size()) {
			while (node->left){
				if (explored.find(node->left) == explored.end()) {
					node = node->left;
					result.push_back(node->val);
					res.push_back(node);
					explored.insert(node);
				}
				else break;
		    }

			if (node->right) {
				if (explored.find(node->right) == explored.end()) {
					node = node->right;
					res.push_back(node);
					result.push_back(node->val);
					explored.insert(node);
				}
			}

			if (!node->left && !node->right)
				res.pop_back();

			else if (node->left || node->right) {
				int c = 0;
				if (node->left) {
					if (explored.find(node->left) != explored.end())
						c++;
				}
				else 
					c++;
				if (node->right) {
					if (explored.find(node->right) != explored.end())
						c++;
				}
				else
					c++;
				if (c == 2)
					res.pop_back();

			}
			if(res.size())
				node = res[res.size() - 1];
			
		}
		return result;
	}
};