leet530：二叉搜索树的最小绝对差

static TreeNode pre = null;
static int res = Integer.MAX_VALUE;
public static int getMinimumDifference(TreeNode root) {
    inOrder(root);
    return res;
}
public static void inOrder(TreeNode root) {
    if(root == null) {
        return;
    }
    inOrder(root.left);
    if(pre != null) {
        //res = Math.min(res, root.val - pre.val);
        res = Math.min(res, Math.abs(root.val - pre.val));
    }
    pre = root;
    inOrder(root.right);
}