leetcode 99. Recover Binary Search Tree

Two elements of a binary search tree (BST) are swapped by mistake.

Recover the tree without changing its structure.

Note:
A solution using O( n ) space is pretty straight forward. Could you devise a constant space solution?

class Solution {
public:
	void recoverTree(TreeNode* root) {
		//vector<int>re;
		vector<TreeNode*>que;
		TreeNode*pre = NULL,*n1(NULL),*n2(NULL),*n3(NULL);
		que.push_back(root);
		bool f = true;
		while (!que.empty()&&(n2==NULL))
		{
			if (f&&que.back()->left != NULL)
				que.push_back(que.back()->left);
			else
			{
				//re.push_back(que.back()->val);
				TreeNode*n = que.back();
				que.pop_back();
				if (pre != NULL)
				{
					if (n->val <= pre->val&&n1 == NULL)
					{
						n1 = pre;
						n3 = n;
					}
					else if (n->val <= pre->val)
						n2 = n;
				}
				if (n->right != NULL)
				{
					que.push_back(n->right);
					f = true;
				}
				else
					f = false;
				pre = n;
			}
			
		}
		if (n2 == NULL)
		{
			int t = n1->val;
			n1->val = n3->val;
			n3->val = t;
		}
		else
		{
			int t = n1->val;
			n1->val = n2->val;
			n2->val = t;
		}
	}
};

accepted