二叉树内两个节点的最长距离

private static class Result{  
    int maxDistance;  
    int maxDepth;  
    public Result() {  
    }  
  
    public Result(int maxDistance, int maxDepth) {  
        this.maxDistance = maxDistance;  
        this.maxDepth = maxDepth;  
    }  
}  
    int getMaxDistance(TreeNode root){
      return getMaxDistanceResult(root).maxDistance;
    }
    Result getMaxDistanceResult(TreeNode root){
        if(root == null){
            Result empty = new Result(0,-1);
            return empty;
        }
        Result lmd = getMaxDistanceResult(root.left);
        Result rmd = getMaxDistanceResult(root.right);
        Result result = new Result();
        result.maxDepth = Math.max(lmd.maxDepth,rmd.maxDepth) + 1;
        result.maxDistance = Math.max(lmd.maxDepth + rmd.maxDepth,Math.max(lmd.maxDistance,rmd.maxDistance));
        return result;
    }