class Solution {
    private int preNodeValue = -1;
    private int result = Integer.MAX_VALUE;
    public int getMinimumDifference(TreeNode root) {
        inorder(root);
        return this.result;
    }
    private void inorder(TreeNode root) {
        if (root == null) {
            return;
        }
        inorder(root.left);
        if (this.preNodeValue != -1) {
            
            
            this.result = Integer.min(root.val - this.preNodeValue, this.result);
        }
        this.preNodeValue = root.val;
        inorder(root.right);
    }
}