class Solution {
    int res = Integer.MAX_VALUE;
	TreeNode last = null;

	public int getMinimumDifference(TreeNode root) {
		if (root.left != null) {
			getMinimumDifference(root.left);
		}
		res = last == null ? res : Math.min(res, Math.abs(last.val - root.val));
		last = root;
		if (root.right != null) {
			getMinimumDifference(root.right);
		}
		return res;
	}
}