【leetcode】Recover Binary Search Tree

//store disorder nodes, most of the number is 4.
vector<TreeNode*> outorder;
//point to the last node of inorder sequence
TreeNode *pre = NULL;
void visitInorder(TreeNode* root){
	
	if(root == NULL)
		return;
	
	if(root->left)
		visitInorder(root->left);
	
	if(pre == NULL)
		pre = root;

	else if(root->val < pre->val){

	    outorder.push_back(pre);
		outorder.push_back(root);
		
	}
	//move pre to the new tail.
	pre = root;
	if(root->right)
		visitInorder(root->right);
}

void recoverTree(TreeNode *root) {

	if( NULL == root )
		return;

	visitInorder(root);

	if(outorder.size())
	{
		swap(outorder[0]->val, outorder[outorder.size() - 1]->val);
	}
}