Recover Binary Search Tree

class Solution {
    TreeNode a = null, b = null, pre = null;
	public void recoverTree(TreeNode root) {
		if (root == null)
			return;

		recover(root);
		TreeNode temp = new TreeNode(0);
		temp.val = a.val;
		a.val = b.val;
		b.val = temp.val;
	} // end

	public void recover(TreeNode root) {
		if(root==null) return ;
		
		recover(root.left);
		if(pre!=null && pre.val > root.val) {
			if(a==null){
				a = pre;
				b = root;
			}else{
				b = root;
			}
		}
		pre = root;
		recover(root.right);
	} // end
} // end class