public class  {
    int diff = Integer.MAX_VALUE;
    
    // 这样访问最小值的时候，Integer 就是 null 
    // 这样可以跳过这个case
    Integer min = null;
    public int getMinimumDifference(TreeNode root) {
        
        if(root == null && root.left == null && root.right == null) {
            return 0;
        }
        
        dfs(root);
        
        return diff;
    }
    
    private void dfs(TreeNode root) {
        if(root == null) {
            return;
        }
        dfs(root.left);
        
        if(min != null) {
            diff = Math.min(diff, root.val - min);
        }
        
        min = root.val;
        
        dfs(root.right);
    }
}