Binary Tree Preorder Traversal（迭代写法）

题目： Given a binary tree, return the preorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3},

   1
    \
     2
    /
   3

return [1,2,3].

Note: Recursive solution is trivial, could you do it iteratively?

算法思想：参见点击打开链接

vector preorderTraversal(TreeNode *root) 
{
    TreeNode *p = root;
	stack record;
	vector res;

    if (root == NULL) return res;
    
	do 
	{
	    record.push(p);
	    res.push_back(p->val);

	    while (true)
	    {
	     
       		if (p->left != NULL)
    		{
    			p = p->left;
    			record.push(p);
    			res.push_back(p->val);
    		}
    		else if (p->right != NULL)
    		{
    			p = p->right;
    			record.push(p);
    			res.push_back(p->val);
    		}
    		else
    			break;
	    }

		while (!record.empty())
		{
			p = record.top();
			record.pop();
			
			if (record.empty()) break;
			
			if (record.top()->right != NULL && p != record.top()->right)
			{
				p = record.top()->right;
				break;
			}
		}
	} while (!record.empty());
	
	return res;
}